If you are referring to Canon's Photoshop plug-in for printing to Canon printers, then you'll need to look to Canon to develop software that will work in Lightroom, not the other way around...Lightroom is designed to use the OS level print pipeline, not some proprietary plug-in technology. Canon isn't known for making great software...but they do develop OS level printer support. That would be the support you could expect Lightroom to allow soft proofing for when it happens.
I also wouldn't read too much into Adobe and Epson's relationship. Adobe has a healthy relationship with the three main print manufacturers. Adobe, Apple, MSFT, Epson, Canon and HP all work together on a project aimed at providing a best print experience. Ironically, the one entity that does the worst in providing a stable print pipeline is Apple. Where ColorSync used to be thought of as a plus, it now seems to cause far more problems than it solves.
In my experience, people who don't know HOW to use soft proofing tend to discount it's usefulness. Soft proofing in Photoshop works very well if (and only if) you have a well profiled display, a well profiled printer and the knowledge of how to use the soft proofing setup to predict not only the final color but also dynamic range of the print.
To the extent that soft proofing can predict color, today's printers can often print colors outside of the gamut of a display (the Epson 79/9900 printers can easily print colors outside of Adobe RGB let alone sRGB). So, determining the rendering intent and then predicting the impact of the dynamic range of the final print is where soft proofing becomes useful. Actually seeing the full gamut of the print isn't as successful because of the limited gamut of the displays particularly if your working space is Pro Photo RGB...but if you wish to extract the optimal results from your images when printed to paper, you really only have two choices; trial and error and constant fiddling to get a final print or soft proof to reduce the numbers of shots you take at getting the optimal print. Even with soft proofing there still needs to be real proofs made to confirm the prediction of the soft proof and finesse the final print appearance for the substrate and ink. Soft proofing just cuts the workload at arriving at an optimal result.
Thank you, Jeff, for the well thought out reply. You provided me a more complete perspective in the scheme of things as goes software development and third-party plugins.
I do have a well profiled display and printer, no doubt it is an issue of user skill. That said, I am very happy with what I get when printing and I'd say it is well within 95% or better of what I see on the display. It is predictable, and that seems the key... predicability. Along the lines of predictability, I am in agreement with PeterAit:
In LR, my "soft proofing" occurs in my head. My experience is that getting the print to match the screen image (assuming your color calibration is set up properly) almost always involves 2 things: a little more contrast and a little more brightness. The resulting print may not be a 100% precise match, but it's almost always close enough as to not make any meaningful difference.
I, too, almost subconsciously add extra contrast and brightness, and the final print appearance is otherwise primarily limited by the inevitable differences between display and printer gamuts, these differences being of minor if any significance, IMO.
Edit: I should add, this inevitable difference between printer/display gamuts is minimized in its effect by choosing the correct rendering intent. I use soft proofing for rendering intent determination inside the Photoshop-Canon Print Plugin, my last step prior to clicking the print command.