Hmm... you could try
this. You'll need the original MechAssault, a memory card with a special save on it (either from a friend or you'd need a modded xbox. Which would defeat the whole purpose.), and some programs/data.
Pretty much, you put the mech save on your HD, run mech, and run the save. It'll launch Evo-X, and you go from there. Luck.
Oh, and this way is
currently invisible to Xbox live. When it goes to 2.0, nobody knows. M$ has to know about this exploit by now.