Probably. There's one in the combo and the amps are almost identical inside for the most part other than the power amp and a few other odds n ends like a headphone jack. I don't know why Line6 thought it important enough to not put one on the head units. If you look at the schematic, if you have it, you'll see at least part of it, U23 & U24 the headphone amp, on the main board. I don't know if all the internals needed other than just the jack are in there or not. A decent tech could probably sort it out for you though.
I only have schematics for the head unit but have a combo amp which includes a headphone jack. However I haven't bothered to crack the amp open to trace it out yet. The manual I have doesn't include the headphone output jack PCB either but looking at head amp schematic I have, it looks like all that may be on it is the headphone jack itself so my guess is that you have what you need other than a jack and maybe a switch to mute the power amp if you want while using your headphones.
What the manual say's about it is that there is a headphone sense signal, generated by a contact in the headphone jack, the bit is sensed by one of the DSPs. This flag bit automatically mutes the power amp by muting digital output to the power amp DAC U20 when headphones are plugged in. However this bit is ignored in the coding for the head units. There is another bit which tells the digital stuff that you have either a head or combo amp. This controls some other little fiddly bits like contrast temperature sensing for the displays.
I don't know how important this muting stuff would be to you or what other things maybe different in the coding for the two but it looks like something that could easily be worked around by any competent tech. Say by lifting pin 8 on that U20 DAC, an AK4393 digital to analog converter chip. Line 6 has it stuck low to ground all the time as it's wired now. If you pulled that pin high to 3.3V when a pair of headphones are plugged in it would mute the DAC that feeds the power amp. This would do pretty much the same thing that the combo amp does, just in a different way and in a different place.
Hope this helps. You can search out a link to the service manual for the head amp on this board. I saw it referenced again just the other day. If not let me know. I wish Line6 would go ahead and put it up on their site but so far they haven't for some unknown reason. Just mean I guess.
This was my thought. The main boards are the same in both the combo and in the head so. Why not find the socket where the headphones go and solder up some wire/jack. Why would I need to worry about the power amp mute issue? I could just turn off the speakers (poweramp) in the settings. The only thing I could think is that there might be some kind of headphone buffer board (to make the louder). Is there anything like that? I dont see any kind of "headphone preamp" in the combo/ head sechmatic I have.
I think it's just U23 and U24 for the two channels left and right which power the headphone. The DAC output from U21 and it's amp U18 drives both analog channels left and right, into a buffer chip U14 a TL074, a quad opamp used as a unity gain buffer, which supplies output to the headphone amps U23 and U24 as well as the unbalanced and balanced direct outputs. According to the TI datasheet, the TL084, used for both U23 and U24, also a quad opamp, should have plenty enough oomph necessary to drive a headphone when connected as they have it. The first amp in the package is a buffer/gain block while the other three are running in parallel as unity gain buffers to supply the needed current and sufficient power to drive to the headphone output. Hope that made sense to ya. I think all that's needed it a headphone jack if both U23 and U24 are stuffed and present on the board.
Which schematic do you have? I'm looking at sheet 5 of 8, titled AX3 Main PCB Direct Out / Headphones.
As far as the muting goes, you're right you can either turn it off manually by turning the speakers off or mute the power amp automaticlly like the factory does when you plug the headphones in, by muting the power amp DAC U20 as I was saying. It just depends on which jack you get for the headphone and adding a couple of resistors to do the auto muting bit. There is no significant difference in cost or work really.