I'd really like to go back in time and prevent signatures from being parts of nodes (despite being one of the few people who actually made regular use of this fact for a while and frequently customized my .sig per node). And I may one day get to implementing things so that new nodes no longer have signatures as node contents.

But even if signatures were not parts of nodes, they would still not be permitted to extend into the gaps on either side. They might end up in a separate <tr>, though I certainly think that they might not (and certainly could and likely would remain in the same <td> as the node contents).