I like this a lot, but I also see merit in Alvaro's PERCENT_OPT variadic, which at least avoids the two lists getting out of sync.
Initially, I was going to ask that we have shell-quote-safe equivalents of whatever fixed parameters we baked in, but this allows the caller to do that as needed. It seems we could now just copy quote_identifier and strip out the keyword checks to get the desired effect. Has anyone else had a need for quote-safe args in the shell commands?