I'm not sure if it could be modded in. As for having it as a feature in the un-modded game, I think it really is just a "different" design decision. I mean not better, not worse, just different.

    The main problems I see with both E:WoM and FE appear to both have a root problem of uncontrollable scaling for certain stats.  I.E. Sov. Hps and then huge mana pools that are effectively infinite later in the game.  Basically this prevents the infinite mana pool scenario.  Is it the best scenario, no.  I personally believe a dual mana system, global and tactical, would have been the best but with the current swing of development this is no longer feasible unless it is unlocked.  This and re-allowing stats to be used instead of attack, defense, accuracy, and dodge.  Make those derived stats based on base stats (STR, DEX, ...) would be the best combination in my opinion, but I digress.  For this idea of capped mana would be a way of say making a level 1 Sov have a Global mana cap of say 10 and mana regen of 2 or even 5.  So in a fight he is able to cast 10 mana worth of spells and if he was battled again the next turn would be able to use 5 mana worth of spells. 

     Enchantments are something I have always had a problem with as I think they should just cost a LOT of mana and be permanent except combat enchants.  By having the enchants cost a lot of mana it limits being able to cast them until the global mana pool cap exceeds the cost of the enchant.  Then limit the number of enchants per city/unit but have no upkeep.  This will also go along with the uncontrollable scaling factor I mentioned earlier; +1 of a bonus from an enchant is much easier to balance than +10%.  Several multipliers scale badly versus several +1's.

     For reference I do like what has been done with the game even if some things I liked were removed.  With help this game will be what we all hope it will be the best 4X Fantasy game for a long time.

Once again this is all my 2 copper.

     The numbers given are arbitrary but a finite cap on an item makes for better strategy than just lets get a huge lump before we do anything.  You are right about metal, gildar and other resources and if possible I would put a cap per city on those resources as you need to store them somewhere but once again this is only my opinion.  What would you say would be an appropriate number for a mana cap.  For materials cap that is something that could be based upon city type and size.  Enclaves could even give a boost to global mana cap.  Hmmm, the more I think about it why not not make a hard cap and just make it a cap by buildings so that you are still limited by development but as you said it is still possible to create a larger mana pool.  I would still keep the mana regen high for the initial leveling and eventually mana regen becomes a moot point due to additional mana regen buildings.  Unless you are in successive epic magic battles then you could be hurting.

     For enchants when I mean limiting the total number of enchants I mean only one or at the most two.  If two they have to be of different types to prevent stacking problems.  I am not sure to be honest for unit enchants, as unit balancing is yet to be fleshed out fully in the beta, I am just referring to resource management aspect of the game for mana and as you mentioned materials esq.

Hard caps are not at all fun. If I want to bank my mana for that upcoming war I have planned, then that is a strategic choice I should be able to make.