Given they're both your mods, I get the feeling this should already work, but it does not appear to be possible to apply a banner to a Comforts sleeping bag or hammock.

Mod versions in use, in case it should be working and is a bug that it isn't:

• bedspreads-forge-1.16.4-5.1.0.1.jar
• comforts-forge-1.16.4-4.0.0.3.jar

This is a fairly tedious endeavor, as both the sleeping bag and the hammock would need their own custom textures for each banner pattern. Essentially, double the amount of textures I've already worked on to make Bedspreads work for vanilla beds. I'm not dismissing the possibility, so I'll leave this issue open, but it's fairly low priority and I'm not sure when I'll have the time to get to it.