Hi Mike,
I don't believe there is anything that says you can't use continous HF on steel, it is just not necessary.
Aluminum is one of many "film protected" materials. Al achieves it's corrosion resistance because of a natural reaction between the Al and the O2 in the air, forming a thin film on the surface. Like most film protected materials, the thin layer of protection has a substantially higher melting temperature than the Al alone, and needs to be removed prior to welding.
While grinding or sanding would seem like a suitable means of removing the film, the film returns almost instantanously.
AC (Alternating Current) provides the cleaning action necessary to remove the film, and as the cleaning action occurs under the gas shield, which excludes the O2, the film doesn't reform until the gas shield is removed.
DC (Direct Current) is generally used with steel because there is no film protection that needs to be removed, and hence no cleaning action is required, at least not at the level that AC provides.
Unless your really worried about Tungsten contamination, then you don't need HF for steel. Scratch start( means just what you'd think. You scratch the Tungsten across the surface to initiate the arc) and lift arc ( you touch the Tungsten to the steel and the arc is initiated as you lift off) provide an acceptable means of arc iniation without the need for HF.
GTAW is a rather high heat input process, potentially leading to a lot of distortion, especially on thin gauge body metal. Have you given any thought to GMAW, or more specifically, short-arc(GMAW-S)? GMAW-P(pulse arc) perhaps?
I have personally had good luck with distortion using 100% CO2, .045" 70-S2, 21V, 100A.
Hope this helps
Dale Simonds