How to implement simple value checks in MDG-S

Hi MDG colleagues,

I'm working on a project to implement MDG-S, working with the latest version of MDG.

I've added a number of bespoke fields to various Supplier entity types, and these are filled by the user in the UI.

One of these is a field for a reference vendor, and so should only hold valid vendor numbers, from LFA1.

What is the best way to perform a simple check against the value table of the domain used for the attribute? (e.g. LFA1 in this case)

The options seem to be:

1. Code a BAdI using interface IF_EX_USMD_RULE_SERVICE. I've tried this and it works, however the check runs on every FPM_REFRESH event - there's no obvious way to check if a value has changed. Also the eroneous field is not highlighted.
2. Use BRF+
3. Code a check into the feeder class. It looks like I'd need to look for a value change in the FLUSH method, store any new value, then perform the check in the PROCESS_EVENT method
4. Is there any automated way to perform checks against the value table in FPM?

thanks, Roger