I don't know how useful any of this is, or whether anyone can incorporate this sort of payee naming methodology into the MMEx import process, but I thought I'd post it here and share it.
I've found that a lot of the data from a number of different payees is downloaded from my bank with a transaction number/receipt number/processing register number/etc and so I either end up having to manually change the payee, or (re-)add the category. To this end, I devised a spreadsheet to help make this a little easier for myself (hopefully it's pretty self-explanatory). Basically, now I import my transactions, run the update SQL in my spreadsheet (verifying the data is ok) and then go back to MMEx and reconcile the transactions. The only thing you'll probably need to change is the location of your database file (in the VBA).
Anyway, I hope this is of some use to someone else out there.