I was looking at the signals present on the slot connector of an MSX machine, but can't find any valid option to implement DMA. So, no such an option on these systems?
No, MSX slots do not support DMA
The MSX cartridge slot(s) are strictly memory orientated expansion design.
- Address lines are defined as output only
- Control lines (RD, WR, IORQ,MRQ, etc.) are output only
- Bus management signals (BREQ, BACK) are not present
What Can be Done:
The closest thing to DMA that can be done are the repeating string operations (OTIR/INIR/etc.). With fast devices (like SSD) this will as fast as using a DMA unit. With slower devices it may need blocking by adding wait cycles.
But Some Machines Have Expansion Ports
True, many manufacturers did add an expansion connector to some of their machines. Most common a 50 pin two row pinheader which carries the same signals as the slot - so not really helpful.
Same goes for Yamaha's 60 signal module slot which is present at some other manufacturers systems as well. The additional signals only provide ways to interact with video signals.
Very few, did add a greatly enhanced bus connector which really does feature all signals for bus takeover. For example the Victor HC-90 and Victor HC-95 MSX2 computers had VG96 type bus connector providing even more signals for deep integration. But all of those connectors are extreme machine specific.