Ethics

Imagine that you (a lawyer) are hired as an ethics expert in a legal malpractice case.  You ask for and are given a retainer as an advance on your fees.  Do you have to deposit the retainer into a client trust account and draw on it only as you have earned your fees?  Or can you deposit the full amount into your personal account and simply refund any money that you do not subsequently earn?

One ethics expert has given me the latter answer, on the theory that ethics experts are not retained for their legal services and are thus not subject to the usual rules regarding retainers.  The answer seems plausible to me, but it raises the larger question of which of the ethics rules (if any) apply to a lawyer who is serving as an expert.  I suspect that this issue has been addressed by some authority, article, or treatise (or all of the above).  Can anyone offer citations that address the retainer issue or the broader question of which ethics rules apply to lawyer-experts?