I just want to consider for a moment that part of your goal for spreading the RDD ability bonuses out over more than 10 levels might be to prevent one of the most common uses of those 10 levels: A quick path to more effective Devastating Critical. If that is a consideration, then ability penalties and AB penalties applied through item properties on a skin or persistent effects will not quite do the job, as the toon will still be able to qualify for the feat as if the penalties weren't there.
[Just to avoid any unmerited assumptions: I am not saying that getting Dev Crit early is so hard to do, even if the final RDD ability bump of +4 STR were delayed well past RDD 10. Everyone here knows it's possible with just RDD 4 to build a toon who gets Dev Crit at level 21, even with a halfling! But, having a toon with a DC 25 (DC 31 fully buffed) instant death attack potentially 4 times per round at level 21 is not possible without RDD. IMO, that's one of the reasons the class earns its reputation as overpowered.]
Sadly, Bioware didn't leave us many tools for customizing the RDD ability bumps in a way that respects feat requirements and so on. They could have easily set them up as individual class feats and then module makers could alter the progression with simple 2DA changes.
The lore requirement for RDDs is sort of a joke, since it's a class skill for every class. If one wanted to make RDD a little harder to take, one could add a feat requirement that not every melee build takes, like Blooded (which has some role-play applicability), and perhaps a skill requirement like 8 - 10 ranks of spot (which isn't a class skill for bards or fighters, but which has some RP justification because dragons have true sight) to provide at least some barrier to taking the class nearly effortlessly.
(Yes, that would encourage more Bard/CoT/RDD builds, but at least that is a more substantial trade-off, since that combo requires 10 pre-epic levels of bard, loses WS/EWS, and disallows dumping starting CHA via taking fighter at level 1. Even subbing ranger for CoT incurs costs in terms of lost feats and potential XP penalties.)
BTW, I agree that some bonus to casters would make the class more interesting. Even bonuses to concentration and/or spellcraft (say +1 every 4 levels) would be useful. I don't know how easy it would be to do, but some sort of "Heavy Breather" feat that allowed an extra use of dragon breath once for every 4 levels past 10 and maybe bumped the DC would encourage people to take extra levels of RDD.
You're right. I didn't think of this.
But does it really make that much of a difference?
I mean, taking 10lvls of a different class only to gain one (albeit powerful) feat in return doesn't seem to be overpowerly beneficial. Sure, it is right now a very good strategy, when with these 10lvls also come +8str, +2int, +2wis, +2dex, but if this is taken care of with one of the above solutions, all that a player can gain from getting these 10lvls is DC, which will still be easier to block than if one took it "legitimately" due to to strength penalties.
Is it worth it if one doesn't look for anything else in RDD?
Note that I care especially for class balance at level 40. It would be great if it could be maintained at earlier levels, too, but if that's impossible, I can swallow that.
Edit:
Ah, one more thing I should've mentioned earlier (but forgot, damn me).
My module will have a minimum level restriction of 10lvls for EVERY class (with the obvious exception of PDK and HS classes), to avoid players investing only a few levels of some class to gain some class abilities, even if they don't actually want their character to be this class and do it only for these perks. I have two reasons for doing it, first one being: I firmly believe it ruins the immersion and RP experience. Paladins with a single monk level make no sense, etc. The other reason is that I think it's easier to balance classes this way.
Well, no matter what the reasons, this might change your thoughts on the subject of RDD having it easier with Devastating Critical. If one is left with only 20 levels of a melee class (since 10 are invested in RDD and 10 need to be invested in either bard or sorcerer), this becomes less tempting.
If spells were modified to use RDD levels if they're higher than bard/sorcerer (or the sum of them) caster levels, then RDD class will be useful for creating spellsword characters, but won't be useful in a manner that players use it normally currently (for devastating critical). And I believe that's a good thing.