I do support reform - but I may be convinced that $1M can buy immigration. The main thing I objected to in OP's idea was the extreme possibility (nay, certainty) that a low-amount ($200K and less I say) would result in legitimized human trafficking - people who "immigrate" and then are practically owned by those who paid for their way in. You raise this bar high enough and the possibility disappears.
That being said, the whole idea still sounds sort of sketchy to me. The US (like all industrialized nations) doesn't really need capital - not $1M at a time anyways. What it needs is skilled labour - things that will help the country compete internationally. IMHO skilled labour really ought to be the only form of immigration.
The modern form of indentured servitude seems to be mostly involving illegal immigrants or guest worker type programs that tie a worker to a specific employer. I could be wrong, I'd appreciate any references for a legal immigration system along the lines I suggested that has caused problems (I may be asking in vain -- it may never have been tried).
The tax structure in the US is progressive. If we accepted unlimited immigrants we would end up with many low skilled workers coming here. Regardless of whether this is desirable (I think there's moral arguments to be made in its favor), it's politically impossible.
Placing a bar that involves a fix fee per year, if the issues related to being enslaved could be avoid, would attract exactly who we want -- those who would do so much better in a developed country that it's worth while.
I see it as if you were born in the US, you were freely given this thing, the right to live in the US, that is very valuable on a world market. Why not allow others to buy that same thing?
Then you, too, support reform of the existing US immigration laws (see Kirk's comment above -- investing $1m will get you in).