Like the title states, does fpc have a stack allocating feature for runtime usages?
TTBOMK, it doesn't but, under Windows and, using just a few lines of Assembler, it can be implemented with a moderate amount of care using ntdll's __chkstk (64bit) and _chkstk (32bit) Internally, __chkstk is named "alloca_probe". MS' C/C++ compiler uses it to ensure there is enough space for the locals in a function.
Unfortunately, it does require the programmer to do the necessary manual adjustments to the stack pointer but, using some Assembly- free "tricks" (dirty tricks!) that can be avoided if the function/procedure does _not_ call any other function (except the "trick" function" upon entry.), IOW, a leaf function.