Powder coating (in my experience) isn't that good; you can't use filler, nothing fits together afterwards and any trace of oil or grease leaves pin holes in the finish. Sure, it's tough, but then so's stove enamelling.
I recently had a pair of girder forks powder-coated, and even though every hole was plugged (either with studding nuts and washers, or sacrificial screws) the blasting media found it's way into the spindle area and it took a lot of shifting. The thought of some of that crap being left in the OIF oil tank tube is frightening.
I don't know what the answer is, but for your frame I wouldn't use anything "blasted" which is likely to remain and cause damage. Clearly there are options as so many T140s have been restored. Maybe ask on the Triumph Twin Yahoo group?