Some thoughts.
While filing I-485, you also have to file Personal Biographic form in which, you have to indicate your employment history both past and present. This form itself will indicate to INS that you are not with the sponsering employer. So why not include the current employment letter with the I-485 filing and be true upfront. The entire employment based GC process is based on the underlying LC (and I-140) and AC21 states that this LC (& I140) remains valid with the new employer after 180 days as long as the new job is same or similar. After 180 days, how does it matter whether at the time of filing you had a job offer with the "sponsering employer" or not? How does it change the end-result in any way? The GC approval under the AC21 pretext is unpredictable and nothing can be said for certain.
This is just my view-point and not a legal advise. Please consult a few lawyers.