I don't think it can be achieved with ordinary portastudios cassette decks, but maybe I'm wrong.....
Of course it can, it just takes a little work.
Like you said, you can get close by using just a little outboard eq. Well, keep trying, then! Tape itself lends a little natural compression/distortion, probably the 'softness' you talk about. So like I said, maybe try bouncing a few key tracks to tape. If you bounce your drums to tape, then mix them back in, you will get closer to the effect. Any cheap tape recorder will work to try it out.
The *only* way you are going to get it to sound *exactly* like someone else is to completely bite someone else's style and maybe even steal their equipment. So I'd say, instead of saying it can't be done, experiment yourself with what you can get.
I really think if you bounce a couple tracks to tape, you will start to get close.Don't put so much faith in the equipment. Even if you did have all of RZA's old gear, that gear will not make you RZA.