If you haven't already looked at this, you might consider creating a custom (non-posting) record with the desired fields and then do the approvals on that custom record with the end of that approval process creating a real check transaction. Definitely a bit more complicated, but could work if you're 100% committed to using the check transaction. Easier alternative might be to do these as Journal Entries and just have a custom field or something to segregate out the different types of JEs and approval paths, if needed.