I have done this already, and seen this in Altered Melee Local Tournaments in my place (sadly, the awesome altered melees are not uploaded to Hive).
It is possible to make a Hero available on Altar Tier 3
(Rough Draft, have not checked in W.E.)
- Select the Hero in OE
- Set Techtree Requirements as necessary.
It is possible to also increase Hero Limit when Altar reaches a certain Tier.
- I forgot how to do this, but you need to replace the default melee trigger.
- Retain the default melee trigger, then for every custom hero, add Limit Training of Unit Type trigger.
- Adding the hero in Gameplay Constants doesn't work with limiting, but it is required to get some hero object properties in Melee maps. So add the hero in Gameplay Constants as well.