M2 was a Specialized proprietary metal-matrix blend of aluminum tubing and required a very speficic heat treatment regeime. If you were able to figure out what sort of filler rod would work with their metalurgy (probably not off-the-shelf 6000 or 7000), you would need to re-do the heat treatment post welding or else the heat effected zone from the welding would likely have all the tensile strength of a melted hershy bar. Typical quotes to have a single standardized 6000 series frame heat treated by facilitly with all the special HT equipment are upwards of $500 per frame and the M2 likely would likely need a non-standard heat treatment regieme so likley could be even more expensive. Manufactures heat treat in large batches to save cost, single frame jobs are just cost prohibitive. Most amature built aluminum frames are made from 7000 series material that need much less stringent heat treatment / artificail aging regeime.