Why not just establish a system where a player's strength attribute limits the maximum weight that all of his equipment combined can sustain. Such that, for example, a player with a strength level of 18 might theoretically be able to wear any piece armor he wanted, but he could by no means wear every piece of armor he wanted.
For a player to run around in fully plated armor carrying a large weapon or a sword and shield, that might require a strength of 30 [or more, this would need to be balanced and calibrated by the devs, but i find 30 to be a nice round number], which even at level 35 only gives them an agility of level 10 [for simplicity i'm ignoring skill-attribute conversion], which limits any agility-based skill to 3 at a maximum. Notice immediately that a player wanting to go full-armor could not simultaneously ride on a fully-armored horse.
If such a player wanted to specialize in polearm combat, he would be capped at 138. Whereas a player at level 35 who stopped at strength level 20 would have an agility of 20, and might have to limit himself to medium or medium-heavy armor, but would also have cavalry options that the previous player did not, likewise his wpf would give him a combat speed bonus.
Most players who had reached a maximum level would not choose to max-out on armor simply because of how difficult hitting other players would be with their slow movement speeds [high weight with low agility] and their slow attack speeds [low WM]
Players who want to use a heavy-armor and heavy-horse [warhorse, cataphract, or plated charger] combo will probably not be able to do it unless they find a particular armor combination, they might have to forgo a helmet or use a very light helmet, they might also/alternatively use lighter leg armor. Either of these would make them vulnerable to perceptive players who would see that their tin-can has a weakness and would exploit it.
An interesting alternative to this is to have a system whereby players with heavier armor [in weight terms] without the necessary strength would [due to being less able to support the weight of their armor] be 1. more likely to be knocked down by certain weapons 2. take longer to get up after being knocked down
If the devs coded this properly, most people very likely would not gravitate towards the heaviest armor, horses, and weapons, due to the opportunity costs being too great. All of which could be done without the accountancy of the upkeep system.
So what would the role of Heirlooms be in such a system? I envision heirlooms being equipment that imposes less of a weight penalty on the player [for the heaviest of armors] and-or being MORE EFFECTIVE for the same weight as a regular piece of equivalent equipment [for the less-heavy eqipment]. [and thus serving the same function] In other words, if a lordly set of medium-heavy armor has roughly the same defense capabilities as a regular set of standard heavy armor, and a lordly set of heavy-armor [i'm thinking plate armor] might have slightly more defense, but more importantly, lower weight.
One possible role for upkeep in such a system would be the penalty players have to pay over a period of time if they want their heirloom equipment to have those bonuses. [such upkeep would probably be greater than it is presently, since most players would not be using a full set of heirloom equipment]
If they do not maintain upkeep on the heirlooms, said heirlooms will not 'break' but will revert back to an inert state where their stats are equal to those of their regular counterparts. [an unfixed heirloom sword might be called a 'chipped masterwork sword' in order to differentiate it from a regular sword] This is similar to the principle of many RPGs where players must pay someone or something money in order to have their special weapons 'enchanted'
