I am not an installer or dealer, so I certainly don't have any inside track on this, but back in the "good old days" when many of us did self-installs I did my own, plus the OTA system and a later upgrade. There were no forums then, but I got by and the system worked flawlessly for 7+ years. I have since done an HD upgrade (leased) and for the first time an E*-contracted installer got involved. That went OK, but after I learned a LOT more in this forum I would have been inclined to try what you're attempting to do. Then most recently when I upgraded to my purchased 622 I self-installed that and E* activated it without any questions.
Like Van says, get all the help you can from your available resources, and you might end-up buying the equipment from one of our supporters like DishStore. See if you can get a local E* installer involved who would be willing to "sign-off" on your work to avoid the code and warranty issues that Van mentioned. Obviously that would have to be arranged in advance. If you know someone in the business who trusts your skills, that would certainly be easier. You and the installer can decide how much of the work each of you needs to do and he can "mentor" you and check your work, then assist with the activation. Maybe they'd be OK with you doing most of the grunt work like running the cables and mounting the dish(es), then they'd come back to do the final connections. Their part won't be free since they won't be getting much if anything back from E* on that deal, so the more you do the less the installation will cost you.
I'm really not sure if you will be able to find an installer who would even consider this kind of arrangement, and/or if E* would approve it, but it can't hurt to ask. The worst scenario would be that you purchase and install everything then E* refuses to activate it, so I'd get everything worked out in advance.
Another possibility might be to purchase everything through a local dealer who will also install it and then you can "assist" with the installation to whatever level you and the installer agree to, and the dealer might reduce the price to you somewhat accordingly. E* wouldn't need to be involved in that arrangement up to the point when they activate it, and since the dealer is involved that should satisfy E* as far as the code and warranty go.
Let us know how you make out with this! Many of us are curious...!
PS - Your "works" package would be the HD Platinum at about $100/mo. ($5 more for locals). The 622 will be the "HD PVR" you want and it has the outputs for another SD TV, so maybe you won't need a 3rd receiver. Be prepared to add another $5/mo. "additional receiver fee" if the 622 can't be connected to a phone line. There are also other fees, like the DVR fees (which won't apply at the Platinum level) and I think that second PVR will also add another add'l receiver fee...