What sort of transactions are you entering, that don't have check numbers?

I fiddled around with this for awhile and there may be an acceptable option:

If entering direct payments of some sort, like DD or ACH orders, or electronic payments of some other type, you could set up an alpha-numeric "number" in the register and QB will take it up and start incrementing it like a check number. The number would be basically meaningless, but you could then just leave it in place as it can't be confused with a real check number.

For example, if you enter PMT001 in the check number field and then record the 'check' transactions, then on the next check QB will default to PMT002, and on the next PMT003, and then on up to PMT999, and then it'll roll to ACH1000.

You can replace PMT with whatever you want as your default, like DD or ACH or some such thing, and you might want to start with PMT0001 (four digits) so it goes up to PMT9999 before adding another digit.