First, the solution doesn't scale. If you have 100 people show up grind like mad, the cap would smoothly increase to accommodate the 100 hardcore players as they snowball off into the sunset. If you can't make it dynamic (x% of actives are at current cap, cap rises), maybe the previous "tier" could need be met in the same way by another 100 unique characters for the next cap to raise?
As an example, let's say the first 100 hits R3L0. The cap will not raise unless there are 100 separate characters at R2L0 (ie 200 characters >= R2L0).
You could optionally have the requirement cascade by one or more previous caps (so for the cap to raise you would need 100 >= cap, 200 >= cap-1, 300 >= cap-2, etc). This would give you another lever to play with (population per tier and cascade depth) to figure out what you think is ideal.
Second, stored RP is kind of the worst of both worlds. Earning an IOU isn't satisfying - if anything, it's frustrating. Diminishing returns (even so far as to 1RP per gain) might be a better compromise.