Yeah I'll give you that, maybe the type certificate thing is pushing it a bit.
You gotta understand though that the purpose of this is not so much to tell you or the O.P. that it is ILLEGAL to log time in this way (it isn't), only that it is STUPID to do so. IF he is actually working as a part 121 pilot in the USA, and intends to ever present his logbook to a recruiter ever again, it is a BAD MOVE. It is ALSO to present, for his consideration, the fact that as a former check airman and current Part 121 airline pilot who has worked for both domestic and international carriers, that I have NEVER met any part 121 pilots who log their SIC time as PIC, even though they have full type ratings. Even if they are the designated relief for the sleeping captain, and even if they are line captains themselves and just happened to be assigned relief on that particular trip. They COULD log 61.51 PIC time, but they DON'T, because it is STUPID. They already have ATPs, they are not building time towards a certificate or rating under 61.51 (the intent of the "sole manipulator" rule). It is not a requirement to log PIC time for currency under part 121. So logging PIC sole-manipulator time in those cases would not be doing so under the intent of the law. While technically legal, it would simply be padding the logbook with "Parker time".
The regulations under which said flight were conducted (your F.O.M.) require an SIC. So you are logging both SIC and PIC at the same time, which is also not a good idea and does not make logical sense.
Whether logging real "I signed for the airplane" PIC (FAR part 1) or "sole manipulator" PIC (61.51), the FAA recognizes both as "PIC" time. There is no provision for anything like "PICUS". As a result it gets sticky separating this out between the various entities that actually have an interest in your logbook (FAA, airline people, insurance people). While one could create a second column for "Part 61.51 PIC" or some such thing if building time towards a rating or certificate, you must understand that that is what it is for, logging time towards a certificate or rating. If you are already ATP-certificated and simply padding your logbook with "sole manipulator" time, there is no real purpose to logging it in that manner, and it is not really in the "spirit" of 61.51. Although yes, by the strictest definition it is in fact, legal.
If you already have a full ATP and proceed to log 1500 of your next 3000 hours in the right seat at Virgin America as PIC because "I was PF", you might as well kiss your future job at Delta / AAL / UPS etc goodbye. Pretty decent chance that their people who check your logbook against your application will throw it out as fraudulent when they find that your 3000 jet PIC time was built while concurrently serving as a Part 121 SIC. The interviewers will likely not offer you an additional cup of coffee. You will also not impress an insurance agent AT ALL with a logbook padded with Part 121 "sole manipulator" SIC/PIC time when you try to get signed on to a policy as captain somewhere.
Read this legal interpretation by the FAA if you need more clarification, or if you are like most people who don't have to deal with American bureaucracy and are unfamiliar with the way arcane FAA rules interact with everyday reality:
http://www.faa.gov/about/office_org/...rpretation.pdf