Is there any way of checking how many "immature" (currently maturing) coins there are using bitcoind?
No, but there should be.
Proposal: treat immature coins as starting with -100 confirmations, and modify listtransactions to list immature category=generate coins (with negative confirmations).
There's probably an off-by-one-error lurking there... (will have to make sure the coinbase transaction is spend-able when it goes from -1 to 0 confirmations).