We have recently completed our sign inventory and relationship classes would have been so very useful. Unfortunately Mobile (even the newest 3.0) still forces you into bad database practices, but I guess we have to use what we have.
Because you never really know how many signs will be on a post, implementing multiple fields would be difficult. We used a field in the sign feature class that held a post ID (just a simple incremental number), so every record that had a matching post ID, we knew to be on the same post. The challenge is making sure that the same ID isn't reused by accident. If you have multiple crews in the field, you'll probably want to start each crew at a different number so they don't overlap (ie. crew 1 starts at 1, crew 2 at 501, etc.) We then had to transform this one sign feature class into our production database that uses a pole feature class and sign table (which is, ironically, the setup in the Esri local government data model that is not supported by Mobile...).
It worked for us, but there may be better ideas out there.