Rory, there is nothing preventing that design from being able to work, however there are benefits to mounting the screw underneath the table.
One benefit is improved access, as your design will restrict access to the table on 4 sides. With the screw under the table, both ends of the table should be clear (unless some other part of the design blocks/restricts access).
Another benefit is by using the table frame to mount the screw, you should get a far sturdier mounting for the screw for less materials, as you don't have to build an extra structure above the table.
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For design inspiration at that size of machine, I'd suggest looking at 3040/6040s on ebay. Upscale the general design to the size you're after, but redesign to use supported rails.
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As for friction details for SBR rails, it's not something I've ever personally seen, and might not even be available due to the number of variables that could affect the figures. Have you tried checking the websites for the manufacturers?