A Device for shutting off XP gain would be nice, i would like to keep Chars at a certain level for fleet actions and foundry missions combined with roleplay (wihthout being absolutely underpowered in a miranda) and i bet there are many others who would like to do the same.
Advantages: possibly increased sales of mid-level Ships, extended midlevel "gaming-experience".
This is a brilliant concept -- being able to stay at one particular tier or level seems like it could be very useful to role-players. This doesn't really cost cryptic much, and selling them could even lead to revenue... maybe they will consider it.
I have one caveat though; how does xp interact with fleet projects? Doesn't the same currency you use to level get contributed as 'expertise'? If so, then maybe make it a no-Level token instead, and allow the player to collect all the normal mission rewards, including experience, so that they can continue to contribute to their fleets.