It is sticky, but couldn’t the group form their “bonds” to match the schedule issues invloved? For your drop-in why can’t he play an inherently unstable role? Fewer and lighter bonds frees the group to play on while the drifter is away. Not as rewarding for the drop-in but a result of circumstance.

We have two in our regular group. One plays a messenger type and plays 50% of the time. The other a Monk with an isolated abbey that 75% of the time requires his attention beyond the main story line.