From what you are saying, there is nothing that can be fixed from my end.

It is you that is causing Boinc on your host to fill up with work from Asteroids, it is entirely under your control what cache settings you set,
If you set a cache level that doesn't exceed Seti's capability of supplying work then you'll find that Boinc will pick up tasks from Asteroids in smaller numbers,
Because of your Cache settings you are causing all that 5/100 Asteroids share to be done now, Boinc now won't ask for work from Asteroids until it has priority again,
Remember Recourse share is long term, not short term:

12/31/2014 8:27:20 AM | Asteroids@home | Sending scheduler request: To report completed tasks.
12/31/2014 8:27:20 AM | Asteroids@home | Reporting 1 completed tasks
12/31/2014 8:27:20 AM | Asteroids@home | Not requesting tasks: don't need (CPU: not highest priority project; NVIDIA GPU: not highest priority project)
12/31/2014 8:27:22 AM | Asteroids@home | Scheduler request completed

So saying that there's nothing that you can do is just plain wrong.

Claggy

In every way that I can calculate the resource "sharing" that is happening on my machine, Asteroids is getting 100% of my system resources when both projects have work in my cache.

Sorry, but where I grew up, that's not called sharing.

It should not matter what I have in my cache. If I say I want a 20:1 ratio of SETI to Asteroids work done, then that's what should be happening, if both projects have work in my cache. Anything else is broken.

Each project has it's own rules. If you can't work within the rules and you don't like the rules, you don't need to process data for them. World Community Grid has short deadlines which require me to keep a small queue. I can live with that. I am not sure what the rules are for Asteroids but it appears they want the work back fast and want to be the main project. Sharing works well enough as long as you don't have a prima donna project. What ever you do, don't connect with Climate prediction as they have work units that run for days and take over control of your system.

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Forgot to say in my last.
That friendly projects like Seti and SetiBeta don't play very well when initially set up.

I went back to crunching at Beta after Raistmer's 10th Dec post. And on a 90:10 resource split Beta has still got >50 tasks @ 30mins/task on my computer that haven't been touched since the 24th. They will probably enter "panic mode" when the due date arrives or when the main site runs out of AP's or crashes again.