Most radios,at least from a few years back,had a "priority" setting,so they would flip over to the frequency set as "priority" for a second or so, fairly frequently,& if it is in use would stay on that frequency.
You could set the repeater as "priority" & use a Simplex channel.
The downside to this is if other users are on the repeater the thing will see them & stay on that frequency.
If you are talking on the repeater output,you will probably be overridden by its output signal in the same situation,so you have a similar situation in that case.
If the normal users leave appropriate gaps,they will normally let the bike rider in ,& help him contact you,so if the "priority"is the repeater,you will hear him anyway.