There are industrial Core 2 Quad systems out there with ISA slots. I don't know how much power you need on your acquisition platform, but I think that'll probably fit the bill.
If you're used to dealing with the AVR, and the C language on the AVR, the Arduino already looks awfully hacker unfriendly.
The tolerances for a decent low-order detonation aren't *that* tight.
Your post shows that your knowledge of explosives is terribly inadequate for arguing your position.
The implosion detonation sequence does need to be quite tight timing wise, but it uses shaped charge methods and explosive "lensing" effects via combinations of fast and slow detonating explosives. Those charges produce the implosive shockwave, most of the detotative force is projected inward, rather than outward.
A shaped-charge expert could probably figure out what would need to be done quite handily.
The timing parts are taken care of via wires cut to the same length, reliable switches with known timing characteristics, etc.
Explosive lens design is described in a few places, including a couple of patents and is available via the Internet.
The really nifty part is that you can machine metal "blanks" out of whatever dense inert metal you have handy and test these explosive "lens" configurations in whatever handy blast pit you have available. When the blank comes out of the test process the desired size and shape, you know you've probably got the right configuration.
While gun type weapons are not out of reach, implosion type devices are not out of the realm of possibility.