Ray, ETTR really has nothing to do with HDR shooting. It's a technique preferred by some to get the most out of a single shot. And it should matter little what the shutter speed/ISO combination is as long as you're pushing the histogram to the right edge. If, under your scenario, 1/60 @ 800 pushes the histo to the 'right' spot but 1/30 at base ISO doesn't then it's not a valid comparison of the ETTR methodology.
I think you might have missed the point. It's the shutter speed required for a full ETTR shot which may be the critical factor when making a decision to increase ISO, in the absence of a tripod, or when either the subject or the cameraman is moving.
As you probably know, the auto-alignment of images in Photoshop is now so good, it really isn't necessary to use a tripod when bracketing provided
the longest exposure in the bracketed series is fast enough for a sharp hand-held shot, and provided
your camera has a reasonably fast frame rate.
The longest exposure in a bracketed series will inevitably be longer than the exposure required for a full ETTR (otherwise what's the point?). In such circumstances, if that shutter speed is too slow, bracketing at a higher ISO will not only potentially produce cleaner results (than a single ETTR shot at base ISO), but also sharper results than bracketing at base ISO when that longest exposure is too slow.
In circumstances where there is significant movement in parts of the scene, such as fast moving clouds for example, the automated process of merging to HDR may not produce satisfactory results. In such a situation it may be possible to manually combine feathered selections of sky and foreground (for example), taking the sky from one of the underexposed images and the darker foreground from one of the overexposed images.
By way of further explanation, perhaps I have assumed too much that you are all familiar with the high-ISO advantages of the CMOS design in Canon and Nikon cameras whereby the signal is boosted in analogue form at high ISO before digitisation. This has the effect of resulting in a greater signal-to-noise in the darker parts of an image.
You can test this for yourself. Take any scene with a high SBR (subject brightness range). Make an ETTR shot at base ISO, then shoot the same scene at ISO 800 using the same shutter speed and f stop. The second shot will of course be overexposed, but you should find that the shadows and lower midtones will be significantly cleaner than the ETTR shot at base ISO.
Of course, certain camera designs lack this facility for analogue boost, such as many MFDBs and P&S cameras.