The differences in impedances, levels and other characteristics between bass and headset are the problem with that, ...when plugging into the single 1/4 inch jack of the TBP06 beltpack.
What you suggest will physically "work", but I doubt that you'd be happy with the sound.
In my opinion, the extra cash for the TBP12 beltpack-equipped XD-V55HS would be money well spent, as it has a multi-pin TA4F input connector, which caters for the different characteristics of instrument pickups by having them wired differently to mics.
The headset would then plug in directly, with the full tone and expression of your bass subsequently being input and transmitted via one of these:
No problem with the bass. If you get the correct wiring for the headset then yes it will work, but it will work to the level of the mic so a lot depends on the sensitivity of the mic. Line 6 wireless devices are unity devices and the 35 series has no ability to add gain. Basically they just pass the signal unchanged, so if the signal is too low it will remain too low.