I have exactly the same problem.
I have to manually work around it by creating a custom 'show and require' field.
When I approve the registration I cut and paste their membership number into the user notes field which can only be seen by moderators.
This worked ok, but I then found that each time a user updated there profile they were prompted again for their membership number in the empty field.
To avoid this, when I cut the membership number form the field, I type in 'Supplied, thank you' which stops them being prompted to complete it again.
It's a risky workaround as there's always the chance they may enter it again, but it seems to have worked ok so far.
Would be interested to hear any better solutions though.
Best Regards
Steve
