You can do it with the current Rockbox build:
Browse to the directory you want to record into (eg to <MMC1>), then hit the menu button.
Go to the "Recording" menu
Go to the "Recording Settings" menu
Go to the "Directory" menu
Hit the up/down buttons to change to "Current dir"
Righ click to OK your change.
That's it. When you record, it goes to <MMC1> if thats what you selected.