Screen printing reflective ink on shirts is awesome and you should totally do it. It will make the shirt stand out in a crowd when there is flash photography. However, Their are a few things you must keep in mind when screen printing reflective ink on fabric.
Don’t print flash print.
You should have a good flood and up to two strokes to get a healthy deposit of ink, but flashing between strokes gels the ink and causes less of the reflective bead to be exposed.
Under-base not recommended.
Reflective beads need to stand bright. The base allows the beads to fall and makes for less bead exposure.
Printed reflective ink will never be as bright as a vinyl transfer.
When screen printing reflective you are pushing or pulling the reflective beads through mesh. Half the beads will fall awkwardly and not reflect well. With reflective vinyl transfers the beads are machine laid and therefore controlled so the beads are more exposed and reflect way better.
Screen printed reflective ink is not legal luminosity.
3M currently is the leader on legal luminosity reflective as needed for police, firefighters, and construction. Only downside to using it is that it is expensive.
Print reflective last.
On multiple color designs it is best to print reflective color last to avoid stepping on it and driving the beads down. If you have multiple reflective colors simply print the one with the most exposure last and print wet on wet.