Is there a way to use a formula as the ending cell reference to the AVERAGE
function?

If not, how can I build a formula through concatenation and have it execute?
Here is my example. I use MATCH to find the first column with the value
"Forecast", then back up to the previous column (-1) to get the ending column
for the average function.
=CONCATENATE("=AVERAGE(A1:",ADDRESS(2,MATCH("Forec ast",A2:C2,0)-1),")")