So, first, the new 2015 deals more with seal welds but not really from your standpoint I don't believe. It deals with welds around a common plane for a seal for galvanizing. Might be worth a check for future jobs but probably not relevant for this question.
Next, remember, the engineer can add to, delete from, or in any way they deem necessary modify the applicable code for their job and it's engineered requirements. So, if you have approved shop drawings and contract documents then the placement of seal welds is indeed required.
How you make that happen is totally up to you with the only requirements really being on the finished weld profiles matching Table 6.1 acceptance/rejection criteria; porosity, undercut, overlap, etc. Sizing won't really be an issue other than the process being used will dictate finished weld size a little: larger wire=larger welds, GMAW=larger welds than GTAW, etc.
I believe your welders still must be qualified for the process used. So, some of the things I have stated would tend to indicate that 'YES' you are still governed by the code. But, there are parts of your question that are totally outside the code and are dictated by your contract documents.
He Is In Control, Have a Great Day, Brent
I had to seal weld between 3 inch "stitch welds on a tower one time. I called the engineer and asked him this, "Since it is a seal weld and not for strength of any sort can I run a 6010 downhill since it would be faster?" I didn't have my suitcase at that time.
The engineer thought about it for a second and then said, "yes, we are not concerned with penetration, strength with the "seal welds" we are just trying to keep water out". Later they started using some special caulk, thank goodness because that was a heck of a lot of welding!