Thanks.
Could that custom field be linked to a stored document? I wouldn't have a problem with hiring a 3rd party programmer to do such a thing, but would doing that in anyway infringe on copyrights held by XenForo?
The fact that one can't proceed with membership unless that option is selected would suffice if the TOS was fixed. Since those terms can be re-written at any time, the user could simply claim that the current terms weren't the ones they agreed to on sign-up.
With so many in society being eager to sue for any reason, I don't believe I'm being unreasonable with this concern.