The more desperate you are, the easier it is to exploit you and ideally you will have better reasons than 'it might get me more clients' for shifting a boundary so dramatically.
It's a 'what's the worst that could happen?' thing, and here the answer is 'quite a lot of bad things'.
A search here will reveal a number of issues with car meets. There's the lack of control - you have no idea where you're going to end up and who will be around. The latter shows the privacy issue - do you want to provide a free show for person or persons unknown / have someone (a police patrol?) banging on the window? If shit hit the fan, you don't know you can get out - even if you're not moving, some car doors can't be unlocked by passengers. On top of all that and more, they're less comfortable than even Travelodge floors...
If you're determined to try, then at the very least you want full ID off him, and someone knowing what the car registration is and when to expect to hear from you again. (They don't need to know that cash for sex is involved, this is basic dating / work safety.) He needs to be in no doubt that someone knows who he is and will call the police if you don't get back in touch by the agreed time.
Having said all that, if your marketing says 'no car meets' then someone who asks about them should go into the 'block and ignore' list just for asking. What's his stated reason for preferring them?