I have now added support for saving and loading via the 2000-baud tape interface found on the Blue RAM expansion boards.
Simply plug in the necessary 4K, 16K, or 32K Blue RAM expansion board as normal (-exp blue_ram_4k, etc.) and you will be able to mount a tape in WAV format for loading, or create a new WAV for saving.
I have uploaded an updated single-driver 64-bit Windows build of MAME containing these changes here:http://polygonal-moogle.com/mame/astrocade/astro64_190317.zip
Once you have evaluated the progress so far, kindly send me a private message on these forums and I will respond with the e-mail address associated with my PayPal account.