Are there any Visual / Rapid Application Deployment implementations of APL?
If not, what would be needed to implement such a tool?
If it doesn't exist, what would the libraries/interfaces look like for handling standard GUI controls and system calls (file handling, etc)?
APL2000's APL+WIN system claims to be a RAD product. It surely includes extensive support for "handling standard GUI controls and system calls". So does Dyalog APL.
I suppose what systems count that depends on what you think of as "RAD" and "Visual". Ther is also a product called VisualAPL but I haven't used it.