Having fallen in love with the square format – the LX3 helped a lot as it offers it as a native format – I continue using it under much more difficult circumstances with the DSLR. There are two marks for the square in the viewfinder, visible but not really helpful when it comes to aligning lines with the frame borders or corners. Currently I lean towards slack framing, offering enough leeway to position a rectangular frame during postprocessing, but this is a not really satisfying as it is quite imprecise. And I loose a bit the feeling for the image when not beeing able to use the SLR viewfinder as the tool it was meant for. Given that my DSLR in many other aspects allows to achieve fine results, an adjustable and configurable viewfinder (together with a much better rendering of shallow DoF) is at the top of an imaginary wish list.
Have you considered put a square mask in the viewfinder?