I have had those same idiotic request from some clients even though I know that the difference will be invisible. I scale up the images (5dmk3) strip the metadata and deliver thumping huge 16 bit tiffs and they are delighted. I also notice that a client recently supplied me with iStock images that were shot with a 7D and the kit lens! They got the job done (24x36 poster) but they were not as crisp as I expected of a stock agency and really got me thinking about the MP debate.
As far as high MP camera, I shoot for agencies and I actually catch flack from them for only shooting 22MP and I have probably lost jobs because I don't shoot medium format. I much prefer to shoot 35mm body. So if I can have a 30+MP camera I'll be quite happy. The agencies will probably still have something to complain about because they are MF snobs.
If your client wants to see you using high MP gear and is willing to pay a premium, just rent it, bill them and call it a day.
I actually work for a large health care provider. I am "their" photographer. Even have a contract. They want to use me for everything but they work with outside ad agencies and one of them resist using me with all their might. And one of the excuses they give my company is the resolution of my gear. This is obviously just an excuse to use THEIR guy.
I'd even go out on limb and say that agency has tried to sabotage shoots before just to get out of using me.
It really irks me because 95-100% of Victoria Secret catalogs, posters, billboards are shot with Canon gear and I doubt anyone is complaining about Russell James' resolution.