Reviving this topic after going OT about filters with binocular telescopes in another thread.
The Pentax XW 1.25 inch focuser barrels are too long to work with normal filters -- not enough back focus. One possibility might be a filter that threads inside the 1.25 inch extensions. Not sure anything like that is actually made, though something could probably be machined.
Kevin at Oberwerk followed up with me to say that the XL22 and XL14mm eyepieces have shorter 1.25 inch extensions and will come to focus with filters in the OB XL BTs. So I am going to try that.
The Baader UHC-S filters mentioned earlier in this topic seem promising for binoculars.