Hi Justin,
I think this is a great idea. I am highly interested in PTX code generation.
[...Proposal...]
The proposal is nice and shows that you already have a good idea of your project.
Here some ideas how you can further improve it:
1. Milestones / Time line
You already have a two-phase development plan. I believe it would be nice, if you can further split it into a set of smaller milestones. Each could include a short description of what you plan to deliver, how long its implementation will take and when you plan to implement it during the summer of code. Those milestones could be sorted into the time frame you have for the GSoC. In addition, you could define "Success Criteria" for the midterm/final evaluation.
This will make it easy to see during GSoC, if you are on track with your project and will allow you and your mentor to readjust your milestones if necessary.
When developing mile stones and success criteria, better be conservative and only add items you are confident you can implement during GSoC. You can add additionally a set of "if time permits" milestones, where you put the stuff that is not 100% needed, but that would be good to have.
2. It would be nice to include a description of the examples you have already tested
3. Define the exceptions
It would be good to know what parts you definitely do not plan to implement and best why not (Postponed, impossible, not relevant, ...).
Like this people can understand to what extend your backend will be usable after the GSoC.
4. Phase two is currently a little short
What kind of optimizations do you plan? Have you already an idea or will you investigate this when you get to this point? How much time do you plan to spend on Part II? If it is more than two weeks, it would be good to elaborate a little on what you plan to do there exactly.
So that's all for the moment. As the application was already nice, I just did some conceptually nitpicking. 
Cheers
Tobi