Q: I have a bunch of male JST-RCY connectors lying around and would like to glue one to a board to power some LEDs. Would I be able to plug a cable with female Mini-PV (commonly referred to as "DuPont") connector on the end into that?
A: Yeah, that should work.
The lighting in the above pic is from an LED lamp plugged into a long series of connectors, including the pictured one.
Q: How about the other way: Male Mini-PV/DuPont into female RCY?
A: Yeah, that seems to work also.
Note that the pin spacing/pitch of RCY is 2.5mm, whereas that of DuPont/Mini-PV is 2.54mm (i.e. 1/10″). The pins may also be a slightly different shape: Mini-PV pins are square, while JST-RCY ones are flatter. Those differences don't seem to be enough to prevent 2-pin connectors from mating snugly and securely.