I can think of a safe way to do it without having to really face the HFS before insta-frosting them.
Basically, dig your giant hole down as far as you can be sure is safe, first, but leave a cap on top of it, and a floor at the bottomost point you are certain is safe. Then, flood this tube. As it has a cap on top, and is still technically 'underground', you should be able to fill it with unfrozen water. Next, continue digging one z level below the floor of your tube, and be prepared . . . you should now have a system that looks like this: (Side View)
-----------------
############
#WWWWWWWWW
#WWWWWWWW# - = Open Space
#WWWWWWWW# W = Water
. # = wall
. D = Urist McMiner
.
#WWWWWWWW#
############
#--------------#
#---------D----#
############
#HFS somewhere#
#under here #
Now, as soon as you notice that you have reached the HFS, mine out the edges of the cap on top of your water column. With luck, the central portion will cave in, smash through the 'floor' a few layers above the HFS, and then smash through the ceiling of the HFS itself, opening it up quite nicely. Water should now flood in, and then freeze.
The only problems I forsee with this would be the time it takes to mine out the edges of the tunnel's roof. But that issue can be solved by building a second floor above it, suspended on one support which has a lever to activate it - as soon as hfs is breached, pull lever, boom and frozen hell.