Even with a lightweight bicycle, there looks to be much that could go wrong with that plan. Not least of which is that the bicycle and rack is all unsprung weight being supported on the swingarm mounted mudguard. I don't see what protects the tail light from being scraped when you hit a bump. Probably the rack and bicycle would just disappear on a hard bump.
Dude posts his work in progress pic (and can't even do that much correctly, but wait...it gets better) and asks for opinions. Now I'm sure most of you will find the problem right away, some might take a little longer but it's plain as day...and don't sweat it if you can't find it as it gets divulged in the conversation that ensues. Anyways, the "pic" as posted:
...and this text string begins:
Wow. This is the genius building the bike and he didn't catch that...
it might could maybe work, there are actually 4 of the thinner tubes, and since it's a hard-tail that part is under tension not compression, so maybe, but, yeah I'd expect to later see that bit distorted and some gussets welded on to try to save it, lol