Solved.
I created a new outbound route that has only the trunk I am trying to reach in the trunk sequence. I created a dial pattern that would uniquely select this route. In this case a prefix of 99 and a match pattern of "X." I moved this route to the top of the route tree.
So if anyone prefixes their number with 99, it will be routed out this trunk and FXO port.
To get a dial tone, I've created a shortcut on my phones of 99 plus the number of the trunk. Viola! PSTN dial tone.