I don't believe that head shox will take a plug up the end of the steer tube. The suspension is in the head tube after all.
The canti studs on the fork on the left look like they unscrew. If so you can make up a plate adapter that looks similar to an old style brake booster. Or for that matter if you can find an old brake booster you could use it as your arch over the tire to mount your fender onto. I believe that once the studs come out the thread on the studs and in the holes is something like 8-1.25. Just find a couple of short bolts and cut them to the length you need to mount the brake booster or booster like arch you make up from some plate aluminium.
The good news is that 1/8 or 3/16 aluminium cuts just fine with a wood cutting jigsaw. Then file or sand down the rough edges and you've got a fender mount arch.
I may be remembering wrongly but weren't the Headshox setups 1.25 inch? Does your Hooligan have that big a head tube?