Took me some effort to dig this thread up, but just thought I'd follow-up on this so you guys know how it turned out.
I bought a relay like the one above and wired it into my control panel, finally got everything wired up and did a dry test last night (no 220V, just BCS 5V and 0VDC).
The DPDT relay works great as an interlock, with the BCS I turned both elements to on and am not able to energize both element SSR at the same time.
On a side note, it was really cool to do that test and watch the leds on the SSRs and the DPDT relay light up as the coil is energized. Everything worked as intended, so I'll be doing a 220V test, then a wet test this week - brewing thursday or friday for the first time in a long time.
Also, the BCS is really cool. I used the probes from brewershardware and plugged in his coefficients. When I tested them against a digital probe from my kitchen they're 5 degrees lower on the hot side (hot water from my water cooler) and 5 degrees higher on the low side (ice water). Need to grab all my thermometers and see if my kitchen thermometer is off or the BCS probes are off.