Well, I would strongly recommend you talking to a lawyer. I understand your situation in which someone is sponsoring your GC but not really hiringing you.
Here is what I think (I can always be wrong). If company A files your 140, it means that they have a need of you in their company. Based on this need your 140 will be approved, so once the 140 is approved and you get your EAD, you have to start workign for company A. If you do not work for company A, then the adjudicator will think that there was no reason for company A to file your 140 in the first place, and he can create some trouble. AC21 allows you that due to current needs of the economy, if your 140 is apprvoed, but you want to change your job, you can do so after 180 days. They have the 180 day catch because they want to be sure that company A filled your 140 for legitimate reasons.
I know BCIS does a FBI and IRS check, and they will know that you are not on company A's payroll, instead you are on company B's payrool, it would be fine, if the payroll date is after 180 days of AOS Notice Date, but if it is before that, then you will get into trouble.
With the current situation, and BCIS being serious on taking all necessary steps, I would strongly advice you to talk to a good lawyer regarding your situation.
I have heard about one case in which a guy's friend's company filed his 485, but had never hired him. They had just done it so that he gets his GC. He was on some other company's payroll. He was busted and was in lot of trouble.
Let me also remind you that BCIS will most probaly require to produce pay checks, tax returns, emplyment letter and other documents to verify your job status.
Don't try to make a decision based on comments from me and others, you don't want to screw around with bcis these days.