Perhaps not in the exact way you mean... but I have a related idea. It sounds like you want certain players to be free to earn more than others. Rather than having capped versus uncapped, you could stagger the rate of income.
I actually run a Forum RP myself, and I set up 6 levels of paychecks (7 if you count "none"). What I did was create a Usergroup in XF to correspond with each. Each user in the RP, if they get a paycheck, has just one of these as a
secondary usergroup. Then, in the Paycheck event, under Event Settings, I set each paycheck event to only apply to one usergroup.
So my setup was this:
Registered = No paycheck
Registered - Pay Grade 1 = 100 currency
Registered - Pay Grade 2 = 200 currency
Registered - Pay Grade 3 = 400 currency
Registered - Pay Grade 4 = 600 currency
Registered - Pay Grade 5 = 800 currency
Registered - Pay Grade 6 = 1000 currency
My screenshot shows a bit more. I give a small starter bonus of 500 currency when a new member joins, and users can also pick up small bits of currency by writing in the designated RP nodes (not just the general chat and such). It's also set up so that players can give currency to another player ("Donate") or transfer one currency to another ("Transfer").
If you don't want to stagger income, but just want capped vs uncapped, the only thing I can think of would be to use Taxation or Interest set with the Minimum currency at the 'cap' and only applying to a usergroup for the members you want capped. This isn't perfect, and
@DragonByte Tech would have to confirm if it would even work this way, but if it does what I think, it would only subtract from users who are over that 'Minimum' and stop subtracting when they go below it again.
Idea #3, however: make two currencies. The regular one, Credits, caps at say 5,000. Then make a big-boy currency, Gold, that specified members can earn. You can sell the same items for different currencies in the shop (for instance, a user upgrade that costs 1000 Credits might cost only 10 Gold), or just have some items available with Credits and more desirable items available with Gold.