I found this in CoreAbilities.xml - you might try messing around with the <Value>100</Value rating. Perhaps <Value>200</Value> means -30% to accuracy [or -7.5%]?

    <AbilityBonus InternalName="MaulAbility">
        <AbilityBonusType>Unit_Design</AbilityBonusType>
        <AbilityBonusOption InternalName="Maul">
            <DisplayName>Maul</DisplayName>
            <Description>Unit continues to attack until it misses, each attack has a cumulative -15% to Accuracy</Description>
            <Icon>Ability_Maul_Icon.png</Icon>
            <GameModifier>
                <ModType>Unit</ModType>
                <Attribute>AdjustUnitStat</Attribute>
                <StrVal>UnitStat_MaulChance</StrVal>
                <Value>100</Value>
                <Provides>Unit continues to attack until it misses, each attack has a cumulative -15% to Accuracy</Provides>
            </GameModifier>
            <AdditionalTrainingTurns>60</AdditionalTrainingTurns>
            <CombatRating>100</CombatRating>
            <IsCombatAbility>1</IsCombatAbility>
            <Type>Aggressive</Type>
            <SupportedUnitModelType>JuggernautMale</SupportedUnitModelType>
            <AIData AIPersonality="AI_General">
                <AIPriority>5</AIPriority>
            </AIData>
        </AbilityBonusOption>
    </AbilityBonus>

No I'm pretty sure that that's the chance of the Maul ability triggering when you attack.  (So if you want Maul to trigger less often you could probably lower that, but I don't think raising it would make it more effective?)

I know if you manage to look at the effects of a unit that's in the middle of mauling you'll see an effect called "maul debuff" but I'm not quite sure how you'd change that.

Here is the effect that reduces the accuracy by 15% each successive attack. 

   <SpellDef InternalName="MaulDebuff">
        <DisplayName>Maul Debuff</DisplayName>
        <Description>Effect after a successful Maul attack</Description>
        <IconFG>Ability_Maul_Icon.png</IconFG>
        <SpellBookSortCategory>Unit</SpellBookSortCategory>
        <SpellBookSortSubCategory>UnitCurse</SpellBookSortSubCategory>
        <SpellType>Tactical</SpellType>
        <SpellClass>Defensive</SpellClass>
        <SpellSubClass>Debuff</SpellSubClass>
        <SpellTargetType>Self</SpellTargetType>
        <HideInHiergamenon>1</HideInHiergamenon>
        <IsCastable>0</IsCastable>
        <GameModifier>
            <ModType>Unit</ModType>
            <Attribute>AdjustUnitStat</Attribute>
            <StrVal>UnitStat_Accuracy</StrVal>
            <Duration>1</Duration>
            <Calculate InternalName="Calc" ValueOwner="CastingUnit">
                <Expression><![CDATA[[UnitStat_Accuracy] * 15]]></Expression>
            </Calculate>
            <Calculate InternalName="Value">
                <Expression><![CDATA[[Calc] / -100]]></Expression>
            </Calculate>
        </GameModifier>
        <AIData AIPersonality="AI_General">
            <AIPriority>5</AIPriority>
        </AIData>
    </SpellDef>

If you want to change this for different weapons and other such items you will have to do a little bit of background making for particular weapons. Use some background stats as Heavenfall has set up the foundation in that sense. Then your calculations will go a little more indepth as now equiping items will adjust stats and reduce stat when un-equipped. (UnitStat_IsShield) is kindof the idea.

So if you want axes to loose more accuracy, then you would have to add a UnitStat_IsAxe to the game

        <GameModifier>
            <ModType>Unit</ModType>
            <Attribute>AdjustUnitStat</Attribute>
            <StrVal>UnitStat_IsAxe</StrVal>
            <Value>1</Value>
         </GameModifier>

Then in your calculations above you need to change it to say something like this

             <Calculate InternalName="Calc" ValueOwner="CastingUnit">
                 <Expression><![CDATA[[UnitStat_Accuracy] * 15]]></Expression>
             </Calculate>
             <Calculate InternalName="Calc2" ValueOwner="CastingUnit">
                 <Expression><![CDATA[ ([UnitStat_IsAxe] + 1) * [Calc]]]></Expression>
             </Calculate>
              <Calculate InternalName="Value">
                 <Expression><![CDATA[[Calc2] / -100]]></Expression>
             </Calculate>

This will then double the accuracy penalty with anyone who is carrying the axe. Of course you can change things in a whole lot of ways, this is just one of them.

The value in your ability stat is the percent chance that maul will actually take place, in the above 100% of the time.

Something like this.

    <UnitStatType InternalName="UnitStat_IsAxe">
        <DisplayName>Is Axe</DisplayName>
        <Description>Is this unit wielding an axe?</Description>
        <Hidden>1</Hidden>
        <DefaultValue>0.0</DefaultValue>
    </UnitStatType>