Michael,
Thanks for the reply. Its very disappointing that they don't support basic math functions - what could be the motivation for that? Anyway, i was able to implement my own (hacky!) square root calculator based on the Nth Root Algorithm (http://en.wikipedia.org/wiki/N-th_root_algorithm). Code snippet below for anyone who might be interested:
Calcuate the square root. Based on the http://en.wikipedia.org/wiki/N-th_root_algorithm
set guess [expr {$targetValue/2}]
for {set i 0} {$i < 32} {incr i} {
set guess [expr {$guess + $targetValue/$guess}]
set guess [expr {$guess/2}]
}
set answer $guess
Obviously this doesn't have any special-case handling (1, negative values, etc) and isn't very performant but it works well enough for my use case.