I missed them too, when in a game, I only found the first hero... Since then, I play with max number of heroes in world options.
I suggested in a previous post that, instead a spell of Summon Hero, give us a scroll of Summoning. The scroll could be a unique item, attached to the Hero technology tree, so u could have a total of 3 scrolls, and so 3 heroes summoned this way (levels 5, 7 and 9), I think it should be enough for a possible lack of heroes.
Another question is, how to make that scroll to cost for its use, and how much. I guess that by a pop-up when using it, telling the costs needed could be fine. So the Sov receives a free scroll when the research finishes, but he can use it only when he can pay the price. The sov might sell the scroll too, if not interested.
And for the costs...any option is fine. Recruiting heroes only costs gildar, so using a unique scroll could cost equivalent in gildar (or maybe some more...) + some mana (for example, hero level x 10 or 20, but not much more). I don't agree with influence...but I could accept a low quantity like the hero level in influence points.
Anyway, scroll or spell, it would be fine to have it. I think that having this option would make more people (me included) to play with normal (or even low) number of heroes in world options, as a way to give a use to the spell/scroll.
The AI should be able to use the spell/scroll too...