>> I'm trying to find a way to open a web page from inside mini vMac (a simple
Mac Plus emulator) where I have several Hypercard stacks running. This emulator
does not allow any interacton with the host OS (X) machine, except for two
little apps which get and put clipboard contents from and to the host machine.

Do both systems have access to the same volumes? If so you could script the
Hypercard machine to write to a specific file and the host machine to read
the file.

You then have a number of options on the host machine:

You could watch the folder containing the file
You could check the modification date on the file periodically (using idle
as per Mark)
You could make the file an html file and simply display it in a browser.

HTH,

ES