I just use the Acrylic sprays at my local auto shop in australia.
But might I sway your opinion, if you are going to use oil on the headstock, why not finish the whole guitar in it. I use Tru oil for all my stained guitars. I like it, but its a personal preference as it takes a bit longer and you need patience. It does give the guitar a bit more of a woody feel rather than the plastic feel of poly - but that's just down to personal preference.
The upside, is its a bit more robust if you knock the guitar finish later. you can just get some more oil and spot finish.
IF you have time, get some scrap pieces of wood and dye them with you dyes and try out each of the finishes to see what works for you.
Edit: Ahh right I see you have oiled, man that's looking awesome!
Further Edit: yeah I oil over my decals all the time. never hurt one yet