Thanks mwf_mmfae !
just to be clear: what we need is something like
extern void (*puart_rxCb)(void *);
extern void (*puart_rxIntCb)(void);
extern void (*puart_txCb)(void);
as found at the top of puart.h
so that we can register our own interrupt handler functions (as in the example from Application note "WICED SmartTM Hardware Interfaces")
// Set callback function to app callback function.
puart_rxCb = my_uart_rx_interrupt_callback;
as well as a function to enable the interrupt.
Looking at the source code of the spiffydriver, there isn't seem to be any interrupt handler analogous to that of the puart.
If you want interrupts for SPI, it seems like you have to use regular GPIO interrupts for SPI transactions.
I'll ask the developers if there's any other way for SPI interrupts.