Can the web IDE configure GPIO that is not used in the Espruino PICO but its available on the chip?

A8/B15/14/etc are connected and able to be changed like any other pin (check out https://www.espruino.com/Pico#pinout) - the circuit diagram just uses labels rather than actual wires so if you search you'll see they are brought out to pins. As far as I know every available pin on the Pico's MCU is available via Espruino.

If you're actually asking:

I'm making a new board with a bigger MCU, can I use the pins on it?

Then yes - but the most sane thing to do would be to compile a new firmware yourself. You could use the peek and poke commands to access extra IO but it'll be slower, and a big faff.