I think that, although DC doesn't encourage it, welding on a hydroformed frame is acceptable. They give this caveat about drilling holes and welding in on 3rd Gen frames in a TSB:
HOLES:
"Holes are not to be drilled in the top or bottom of the frame rails. Holes to mount outriggers, brackets, and supports must be drilled in the web (vertical sides) of the frame rail with the following restrictions:
Hole diameter should not exceed 20mm (0. 75 in. ).
Material between edge of hole and top or bottom of frame rail must not be less than 40mm (1 . 60 in. ).
The minimum edge distance between any two (2) holes must be larger than twice the diameter of the larger hole.
Any through frame fasteners that torque down on both external surfaces of the frame rail must use an Internal spacer/reinforcement to prevent crushing the rail tube.
All holes should be drilled In the frame using appropriate safety precautions.
Avoid drilling holes near the fuel tank, fuel and brake lines and other lines and wires to avoid damage to them.
WELDING:
Prior to welding, the following must he done.
Avoid welding near,the fuel tank, fuel and brake lines or other components that may be damaged by the heat of welding, If R is necessary to remove the fuel tank, lines or other components, do it In accordance with applicable service manual procedure.
Components near the welding area which could be damaged by excessive heat must be removed or adequately shielded.
Disconnect the battery(ies)
Precautionary measures should be used to prevent electrical system components or wiring damage.
Frame E-coating must be removed from the weld area prior to welding. Use proper welding techniques to avoid stress risers that may adversely affect frame performance. "
Doesn't sound to me like anything special - just simple guidelines for drilling or welding on vehicles.
I've personally welded something like 90 separate beads on a certain 3rd Gen frame - and drilled around 20 holes. No failures of the frame, no cracking or anything weird around the welds either. I believe in the new frame technology. The fully-boxed frame is nice and rigid. In addition, I'm not sure of the alloy, but, when TIG welding to it, it doesn't like regular welding rods - it likes rods made for welding 4130 - chrome moly. When I say "likes" I mean that if you weld on a 3rd Gen frame, regular rod doesn't merge nicely with the material. What this implies is that the frame is made from a very high grade material - something close to chrome moly. I've welded a lot on 2nd Gen. frames as well. They like regular mild-steel rod.
By the way, I don't want to start an argument about which frame is superior. I'm just stating my experience with one 3rd Gen frame. Over the last 20 years I've severely bent five light duty truck frames. All were of the semi-boxed, C channel type (2nd Gen. design). I've never abused a truck more than the 03 my company owns and the frame is still perfectly straight. Not an argument, just my experience.
Point being that I think it's pretty safe to put some welds on a 3rd Gen frame. Safer, I'm sure, than it is to put your tires below the belt line. :-laf
Cheers,
Kent Kroeker