304 weld passivation?

Yes. The welding has altered the material and left an undesirable, thermally formed oxide film (although quite thin in this case). This film is brittle and probably filled with minute cracks from rapid cooling of the weld metal, whereas passivation films are amorphous and flexible.

Also, if this device requires sterilization with usage, I would further recommend electropolishing.

A medical device manufacturer should have SOPs established for following applicable ASTM & FDA specs.

This will see ethylene oxide sterilization, what would be the benefit of e-polish with regard to sterilization?

The supplier has had some difficulties removing the solutions from the finished assy due to the long small tube ID and space between tubes [at non-welded transitions]. Will a swab wipe of nitric acid based passivation solution have any effect on the oxide film stability or does the thermally formed layer need to be stripped off before re-establishing a more stable layer? Any other ideas for 'spot passivation' techniques to address just the welded areas? [This assy is 8" long with a .009 ID, and has various other tubes welded to step it up to .08".]

Electropolishing smooths the surface on a microscopic scale, making it more difficult for contaminants, bacteria, etc. to adhere, and also more easy to rinse any cleaning solution. Electropolishing generally reduces R_a by 1/3 to 1/2 (For more info on electropolishing, see thread330-50008.

The weld scale on SS cannot be removed by nitric acid alone; it is necessary to add either HF (hydrofluoric acid) or ammonium bifluoride. It can also be removed by electropolishing or abrasively removed. See some cleaning procedures in ASTM A380 or
The more difficult it is for the supplier to remove solutions from the assembly, the more difficult it will be to remove any contamination in use, and hence the more important it has to have a smooth finish.