Personally, I'd polish away if the client wants to wash / clay on their own. Simply have them sign an agreement that states they understand that results may not be optimal due to the fact that they're taking responsibility for some of the steps involved. I've experienced where a customer will call and ask for just an interior shampoo, so I quote an amount based on shampooing and vacuuming (since you obviously have to do both). Every so often, the customer will ask "can I vacuum it myself ?". I say sure, but understand that the shampooing process (extractor) isn't designed to pick up lots of gravel, sand and other soiling (which we get lots of here in the winter since they sand / gravel the roads), so if the carpet isn't vacuumed well enough, the shampooing results might not be optimal. Most customers then simply forget about trying to save money and just pay. The odd one still decides to do the vacuum step themselves, and I've seen vehicles where there was still gravel and sand on the carpets. I just shampoo away and that's that. It's like a Maaco cheap paint job. Might not look the best, but if that's what someone wants to pay for, go for it.
As long as your hourly rate is still good, why not squeeze something like this in on a morning / afternoon before heading to a "complete" correction / detail ? The most important thing is to ensure that the customer knows what to expect, and that they understand that by not having you do all the steps, the result may not be as optimal as it could be.