How hard would it be to allow for custom plugins for ItemValue that simply takes an item link and a string identifier and return a number.
I have a similar addon that I am working on again after a long hiatus. The approach I have taken gets around problems with nonlinear (crit) and capping ratings (hit) by calculating the stats if you were to replace the item you are currently wearing with the item in question. This kind of holistic approach gives you good values even if, say, the item has +20 hit and you are +10 from the cap. It still takes into account the +10 and the other +10 are valued at 0.
My current addon is not nearly as far along as yours (and is mainly set up for feral druids) and I think it would benefit the community better to redesign my addon to work with an existing general addon like this.
I also prefer the holistic approach. the example you give with the hit cap is a good one. When you look at an item in the tooltip, you want to know how it will affect your overall gear value, not how it compares to the item you currently have in the slot. That may sound like much the same thing, but its not with some things, such as +hit when you reach the cap.
This can actually be done with ItemValue formula currently tho:
math.min(CR_SPELLHIT, 202) * 0.8
202 is (from memory) the hit rating for the gear hit cap, and 0.8 is your weighting for (effective) hit rating. for a hit rating of 202, this formula will give a score of 161.6. for a hit rating of 1337, it will give a score of 161.6.
Its even more complicated for affliction warlocks with the suppression talent, you want to value hit rating at a certain amount for how much is needed to reach the hit cap for affliction spells with suppression, and then you want to rate the rest of your hit rating at a lesser value, and rate it at zero when you reach the gear hit cap. The formula becomes:
math.min(CR_SPELLHIT, 77) * 0.8 + math.max(math.min(CR_SPELLHIT, 202) - 77, 0) * 0.4
202 is again the hit cap rating, 77 is the affliction hit cap rating with 5/5 suppression (adjust it for how many talent points you have in suppression). 0.8 is again how much you value fully effective hit rating, while 0.4 is how much you value hit rating that is above the affliction cap, but still useful for non-affliction spells. adjust the weighting to your own scale.
all of that tho, is useless when comparing an individual item, as is done in the tooltip. It is useful only when applied to your entire gear set, since on no single item will you find > 202 hit rating, or even 77. When you look at the ItemValue display on the character sheet, the value is useful.
So the only change I request is (an option perhaps) to make it so that the tooltip can display the difference it would make to your overall gear score if it were to replace the item you currently have equipped (2h versus 1h and rings and trinkets make it a little more interesting).
Is it possible/easy?