6061 T6 is a heat treated wrought alloy. Much of the strength comes from the rolling process and subsequent treating/tempering. If you melt down pure 6061 T6 and pour it, you will not have a casting made of 6061 T6, even if you temper it. It will be a cast alloy similar in composition to 6061, but even that may change if some of your alloying ingredients react with the air at high temp and oxidize or evaporate (which happens).
Now, if you want to experiment with casting an alloy of ths type, here's how to get out cheap;-) Find somebody who does punch work and ask them to save the slugs for you. I have a 5 gal bucket of 3003 slugs that are 3/8x3/4" ovals, punched out at a buddy's shop. It is the perfect size for the crucible, doesn't require breaking up, melts really fast, and being a weldable alloy, it flows and pours like you wish everything would.
Once cast, it machines just like 3003; soft, gummy, and gooey. It makes very solid castings using the lost foam process in dry sand. Much better than scrap waterpump housings and old VW pistons. That may be the best alloy for you to use, actually. It's soft enough to avoid cracking and, as we all know is just rediculously easy to weld. It's not as strong as 6061, but you'll have a really tough time making a set of castings up that will mate well under load with 6061 T6 without the casting being as big as the engine block.
I still think the bike is going to end up weighing more than it would with a steel frame in the end... if it is to retain it's original stiffness and strength. Aluminum is light, but it's not as strong, especially cast aluminum, so you have to use more of it. If you want a nice, light and strong trick frame, build one up out of 4130 and have it plated or powder coated. That's what the old Rickman frames were. I'm still for the titanium frame myself;-)
18x72 L&S, Fosdick 3ft radial, Van Norman 2G bridgemill, Van Norman #12, K. O. Lee T&C grinder, Steptoe-Western 12X universal HS shaper, 16spd benchtop DP, Grob band filer, South Bend 10L