As you noticed, Miva uses its own indexes (MVX), so that would be one issue.
Another would be to ensure that the strict dBase III compatibility was maintained. For example, I use Visual FoxPro every day to analyze and report on my Merchant data, but I can't use it to make updates to the store data because VFP will convert the DBT memo files to its own format.
So you have to be completely sure that whatever you use to modify the DBFs won't change the memo field structure and won't unexpectedly change any bytes in the DBF header.