It is VERY possible for someone to miss a foal through rectal palpation- its happened many, many times. If someone is not extremely experienced, the possibility is large at certain points of the pregnancy that the foal just will not be within their "reach".
OP- go with an ultrasound. I do not like rectal palpations in the first place- only extremely experienced and qualified individuals should perform them, and even then the risk of injury to your mare is more than what I would want to take with my own.
I think you have some misinformation here.
First, every licensed veterinarian is qualified to perform rectal palpation. It is standard operating procedure for working up a suspected colic case. It is wise to be cautious with rectal palpation, yes, but foolish to be afraid of it. Every licensed veterinarian working in an equine practice falls under the "extremely experienced and qualified individual title"
Second, you can literally put your hand on every inch of the uterus until the foal is large enough that it pulls it over the pelvic brim, at which point it is so large, there is no way to miss it. So I can see a very inexperienced veterinarian missing a pregnancy at maybe 40 days on palpation but not with trans-rectal ultrasound.
Third, are you talking about transabdominal ultrasound then, re:too risky for rectal palp? Do you realize that you can't see the foal that way until just a few months before birth? So, if someone thinks they have an accidental pasture bred mare, you're advocating waiting until they are near foaling out to diagnose the pregnancy?!