Multiple email signature work around.
This may be a lame work around, but here goes...
Open one email account, open a new email, type in the various email signatures you want to be able to use, save the email as a draft.
Remove the default email signature from your iPad settings.
Whenever you want to process an email (compose, send, forward) first open the draft, "select" and "copy" the signature you want to use. Then you can "paste" it at the bottom of any email.
You can go back to the draft anytime you want to change to a different signature or after you have had to copy and paste something else.
Hope this helps.