Drilling two 1/4" holes in the trailer's tongue frame, especially if later filled by a bolt, is not going to change the integrity of the tongue frame 1 percent. I would bet that tacking on a cross piece to support other things would have the same result, near zero. With welding, I would think it impossible to take any temper out of the steel to the point where it weakens the frame to the point of failure.
But what do I know, I've done this many times with zero failures. Perhaps, more modern trailers have some type or configuration of steel that I haven't seen. All the ones I've played with have a full box steel frame.