PitBull

The Aquamist failsafe output (at least the one I have:  HFS4 v3) can be set to either go to ground or go high (+5v) on activation.  The characteristic is selected by a jumper setting on the Aquamist control module.
There are several conditions that activate the failsafe:
1. Low fluid detection
2. Flow sensor output is below lower threshold (i.e. pump failure, nozzle clog, meth line break prior to flow sensor, FAV fails closed)
3. Flow sensor output is above upper threshold (meth line break after flow sensor, FAV fails open)
4. Injection system is deactivated by the switch on the flow gauge
If you set the failsafe to go to ground on activation, you could then add a toggle switch onto that wire to short it to ground when you want to trigger the failsafe maps manually.  BUT, you'll want your switch to connect ONLY the Link's DI to ground, while at the same time breaking the connection to the Aquamist failsafe output.  Shorting the AqM failsafe to ground would be a bad idea.
I have my Aquamist failsafe connected to a DI on my Link and I have it set to switch between high and low Boost Solenoid PWM DC tables. 
I do not use it to switch fuel or timing maps.  Rather, I use 4D fuel and ignition tables and have the Aquamist's flow meter output set as one of the axis and RPM as the other.  So as the AqM's flow meter output rises (i.e. more meth being injected), the 4D tables remove fuel and add timing. I am still in the process of tuning this but it seems to be working fairly well.  But I am running 100% methanol only and don't need to switch between 100 or 50/50 like you want to.  You'd need to use fuel and ignition map switching for your purposes.
The one issue I am having to work around is that the AFRs tend to get progressively richer as the meth sprays, even when other variables stay constant (RPM/load/meth duty cycle).  My assumption is that since I have the nozzles just after the intercooler exit and there's about 2~2.5 feet of intake piping between the nozzles and the throttle body, some amount of the meth will wet the intake piping, but once it is wet, then the majority of it flows into the cylinders.  I don't see it as a big problem, however, as the reading I've done on methanol says you can run it richer than gas.  If I can get the AFRs to start out around 11.5 and drop into the 10's during the pull I think it'll be good.