I haven't created a custom attribute before, but I will give it a shot.
A hold on an Ecobee is when you change the temp and you can set it to hold that temp for 2 hours, 4 hours or indefinitely before resuming the scheduled programming on the Ecobee end. It is part of the UI on the thermostat itself.
If you can see that as an event in the driver, it will work in RM. However, if the driver is not aware that it is on āholdā, then you will not be able to use this information in Hubitat.