I'm sorry I've this has been posted somewhere and I've missed it.
Are there any already-written tools out there for monitoring/reporting the balance of multiple addresses (Masternodes)?
You can export the private keys from your individual MN wallets and import them all into a single (normal) wallet.
I think it's also possible to import "watch only" public keys into a wallet but I haven't tried that yet.
Just make sure you don't spend the "wrong" coins (use a different wallet for transactions or use raw transactions).