If the document is designed properly, all that should be required is the application of forms protection after the merge. Of course, if you're worried some changes might be made before the mailmerge is run, you could apply the protection beforehand and use vba to unlock it & run the merge. The problem with that approach, though, is that you'll have to write a lot of code to automate the mailmerge interface as well.

I'm curious, though as to why you believe mailmerge is required. As you say, you:
In that case, what you're asking them to do is to create a mailmerge from scratch for the purposes of your form. You can't even provide the mailmerge field names, as you don't know what they might be called in the data source. And what, when they've done all that, will be produced? If the result is to be nothing more than the population of a few cells in your table, mailmerge is not the way to go - simply give them a form with formfields they can fill in manually. That would require much less effort all round.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]