Many people, myself included, prefer to work in sRGB rather than in ProPhoto. After all, sRGB is still the standard for the great majority of print labs and the internet. We should be able to edit while in the sRGB colorspace.
sRGB is an output color space for uploading to web and mobile devices. That's it. You can fully work in sRGB or any color space in LR! It's color managed. You can soft proof in sRGB and get sRGB numbers back. IOW, there's nothing you're asking for that doesn't already exist.
That is already built into Lightroom CC. On a Mac (no idea about the shortcut in Windows, sorry) , tapping the "S" key switches Lightroom into Soft Proofing mode where you can specify sRGB as the space you edit for.