I don't think it's as easy to fiddle with the system cursors as it once was, I think you have to turn off System Integrity Protections (SIP) if you want to mod the OS now days.
But there are something like 63 cursors in there for various use cases. While people probably should NOT copy Apple's Cursors into their own software's assets due to the potential licensing issues, if the cursor were to already be in .png format (as some of them are, in addition to the PDF), then you could copy them into your stack in order to temporarily use them and then discard that cursor data.
To truly use the system cursor in a more app-context-free system-wide way, you would need some Extension wi/FFI to call ObjC methods....
EDIT: Scratch that, NSCursor works based on what NSWindow/NSview objects are underneath the cursor so there's probably no way to change the system wide cursor other than changing those cursor PDF/PNG files inside that framework bundle.
For xTalk on the web (HyperSim or OXT Emscripten Engine), 'the cursor' is not functional, but you can use a little JavaScript to explicitly set a certain cursor to be used when the mouse is within specific elements (DOM objects) of in the current web page, which is similar to what Apple's AppKit does.
Also, I just found out that 'the clipboard' keyword was added to HyperTalk in HyperCard 2.3:
What's new in version 2.3?
-------------------------------------------------------
Support for PowerPC Macs including the ability to generate standalone
applications which are optimized for PowerPC Macs.
New Button Tasks which allow you to generate HyperTalk scripts without
scripting.
Enhancements to the Color Tools including a new color painting
environment.
Text-to-Speech capabilities. (only for Macintoshes with 68040 or
better microprocessors)
New HyperTalk: speak, the speech, stop speech, the voices, the
soundChannel, stop sound, the clipboard.
Enhancements to many current HyperTalk commands.