Signatures can only be placed on the form statically, so having a repeating (1 to many) dynamic subform with a signature is invalid and is not allowed by designer. I think mostly since a signature field cant be created dynamically, it must exist in the pdf prior to loading.
However, You still can have multiple signatures in a static table as you suggested, making the rows visible/hidden depending on content and cant see why it wont work. But you will need to careful with it. I would presume you will need to make sure you are displaying all rows and then hiding those not in use. Doing the opposite will invalidate signatures. Invisible signatures are not the same as visible signatures and will be invalidated and removed. You should probably only create a new line when needed on a button action, since any hiding will invalidate the previous signatures (since its a form change). You cant lock all fields with multiple signatures and will get errors, so you will have to use form object collections and assign each one to an individual signature ( if fields are involved).
Also, If you are using any LiveCycle server components at all, I'm not sure about the level of support there is for processing multiple signatures. I probably think it would have to be done by a custom component to read the multiple signatures on the submitted xfa data.
There is further background about xfa signatures (a bit older reference) here http://wwwimages.adobe.com/www.adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/digisig_in_XFA.pdf
Sorry, meant to reply to original post. Message was edited by: dohanlon