I personally have no problem with allowing lower tiered players access to class 2s or even opi(plural opus?) for a large token cost. It would drive up the market price on tomes, helping redistribute wealth a bit.
Even if that is not the direction SoD is heading, I have another suggestion. With the way endgame tomes currently work, it is entirely possibly for people to put in all the time and more necessary to progress through a tier yet not get their core tomes. Some players have put in hundreds of hours raiding a zone where their desired tomes are most likely to drop and had no luck. I do realize that this kind of dumb luck is what makes the good tomes that much more desirable, but I think with a loot table that large it starts to get unreasonable.
I would just say use the refuge tokens as such:
normal tomes: 8 tokens
opus 1s: 20 tokens
opus 2s: 24 tokens
class 2: 20 tokens
class 3: 24 tokens
etc
You could potentially add an relatively easy quest here involving a few raid/6man drops to prove a character is on tier and unlock the higher tier tomes. Turn in a drop from spires bosses: unlock opus 1s, turn in CC or Ofgats head: unlock class 2s, etc.
Another way to restrict access to the higher tier tomes, as well as restricting the number of these powerful tomes that enter the world of Dalaya would be to add a Premium Refuge Token recieved for turning in a class tome 2+ or opus 1+ and trade these tokens as such:
class 2: 2 tokens
class 3: 3 tokens
opus 1: 2 tokens
opus 2: 3 tokens
etc
This is of course just a suggestion, and the current system overall has treated me well enough so far, it is more that I commiserate with my guildies who just cant seem to get any of the tomes that are really desirable for their class (or get a class tome 3/4 and have no clue when or if they will ever find the corresponding tome 2)
Edit: Also, before someone says anything about the fact that I buy tomes for tokens. That is completely unrelated to this suggestion. I said these things simply because I think they are a good idea. I buy tomes for token use so I can get 24 cold damage augs. When you have money you don't know what to do with you start doing dumb things like spending ~8k per t5 aug.