missed a few of your questions,
32k sram is history - the microcontroller has more than that.
and an absolute shitload of flash too - 128k in the one i'm looking at.
no buss issues should happen, the iric is 1Mhz, the micro is 20Mhz - it can slave the buss and wait for the clocks.
and as it can run down to about 2.7v weak signals shouldnt be an issue - although that shouldnt happen unless to have a 6inch ribbon cable anyway!
now you asked about if the oric has to wait on sector-reads or if it can go do it's thing - presumably like playing tunez.
forget sectors - that's why the microcontroller has 128k or flash, 64k+ of ram and a 20Mhz clock.
you write bytes, you recieve bytes - the controller worries about sector allocation, erase/write etc.
i dont know yet how best to signal that a byte is ready, either by status-register or interupt.
the cbm machines used an interupt afaik but we dont need to.
now let me be clear,
i have a *very* minimal hardware in mind - leveriging the speed of the microcontroller.
this has never been tried before!
what i want to try is this,
in the past - as you know, you used logic on the bus to decode the addresses of interest.
this then latched a data buffer on the data-bus.
so whatever the hell you tried to do, you had between 3 & 6 ttl chips to do the address decoding first.
i want to try directly putting the microcontroller on the bus - cpu>mcu.
three 8bit ports, one for the data-bus and 2 for the address-bus
then a few bits from a fourth port to watch R/W clock etc.
i'm pretty sure the mcu is fast enough to see the clock, then analyse the 2 address ports and either read, write or ignore the data port.
if not then extra chips will be needed like usual - but if i'm right then you can pretty much read or write *any* address in the oric at will!
i need to find my orig & build a test board - i have the bits to make an sd2iec here.
i can try just intercepting addresses to display status on an led to check if the timing is possible.
if it comes to it the latest version - "arm2iec" will be more than fast enough to do anything you could think up!
but i'd rather use an AVR for cost and simplicity reasons.
this may be worth reading for people who dont know the sd2iec project:
http://www.c64-wiki.com/index.php/SD2IEC
http://www.c64-wiki.com/index.php/sd2iec_(firmware)