Here's a close equivalent of the Voss I use inside of view cameras. The Voss had a spring metal band (rubber cushioned) that grabs the outside of the lens barrel. This Lee equivalent appears to use a rubber band and 100mm filters. Voss made them for 75mm (and I think 100mm) filters. The Voss also had two removable "barn door" style panels for shading when used outside the camera. I removed them for use inside the view camera.
When I purchased my Linhof Technorama, I received a full set of Cokin X-Pro square filters, plus the filter holder that screws to the lens, and I have been very happy with the results. The Schneider Super Angulon XL 90mm f/5.6 is pretty huge as LF lenses go (105mm diameter front element once the fall-off correction lens is screwed onto the front) and I have been very happy with the results.
(as you know, the 90mm lens is extremely wide on the 6x17cm format, comparable to your ~60mm on 4x5in)
Here is an example of the Cokin X-Pro 2-stop Grad ND, in the worst possible lighting (pointed right into the sun) and there is only a tiny bit of flare, and the colours stay quite neutral:
djkloss, to answer your question, yes you can either use a "tobacco grad" filter which wil give a yellow cast as well as decrease exposure, or you can combine a warm-up filter with a Grad ND. With the Cokin X-Pro set, you can slide two filters plus a polariser into the holder, and I've never seen vignetting.
thanks for the info. I fear that there is no room left on the inside of the camera with a bag bellows taking up all the space. I took some shots on Sunday with the adapter ring and one filter at the lake. I just finished developing them so in a few days we'll see if I've learned anything from all of this.