Mihec:
I think that it would be reasonable IF you can use some PVC to make a side mount out a bit from the metallic mast.
I speak not from engineering degree, so please note.
I know from past experience trying to get Boltek to work before the Time Of Arrival stuff came along that metal nearby can and does act as a shield, a reflector and a re-radiator.
I put up a square 10' 'mast' made out of plastic downspout, with a little cap on top of it. All plastic, shielded the antenna (be sure to put a wee dab of dielectric grease on the RJ-45 connector when you put the plug into the antenna to help avoid the dreaded green crud that builds up with the amount of current flowing through the cable (the antenna can get warm so that tells you something)), and keep away from as much metal as you can.
That being said, a mounted antenna on a metal mast is better than no install whatsoever, so try it to see. It looks like your antenna position will not be easy to get to for tweaking and checking.
I had a metal gutter on the edge of the house about 6' away and that was a concern, but nothing I could do about it. Also on a slightly higher garage roof I had panels of solar collectors (the old hot-air type) which were sort of like big metal shields and did have some effect, but less so and it was in a direction where storms receded towards rather from which they approached.
Have fun, but I gave up with severe ghosting problems and support issues. Lightning 2000 author is deceased and while a nice program, got so complex in calibration that I wasted one whole summer trying to get that to work. Astrogenics has announced (if I'm not mistaken) that their reporting network (? StrikeStar) is shutting down. I think their Boltek local software is still available.
I know there are some loyal Boltek enthusiasts and it is an interesting concept before current technology eclipsed the method. There were some lightning detection gizmos made by 3M, I think, that high end private aircraft would install to give an idea of where the lighting was in relation to the plane, and it was basically a very expensive Boltek system with a very low resolution display. You can find them on eBay every now and then but again this has been discontinued.
If I had more time I'd probably futz with my system when storms came by but after giving it the old college try for four or 5 years, I have mine sitting in the corner until spring when I might turn on the old computer to see if it will run one more time.
Let us know how you do, I'm always interested in reports and installation pictures of anything. I hope you get it up (as I said a less than optimal install is better than none) so you can begin to play with it.
Dale