I'm still lost as to why I'd do this when one output is a print, the other is a display.
A lot of people spend a lot of time trying to get their prints to match their monitors. It's not a simple job but it is completely doable providing their monitor gamut encompasses their printed image gamut. It's close to doable even if the monitor gamut is a bit narrower that that of the image.
I can't make an sRGB preview look like a ProPhoto RGB output to an Epson any more than I can get a gallon of water to fit completely into a pint container. If I need sRGB for viewing on the web, I'll send sRGB to that media. If I need a saturated print, using the full gamut of that output device, I'll send it ProPhoto RGB. How the print and the web are connected otherwise is what I'm still not at all clear on.
Your analogy to water containers is flawed. The "gallon" container is never filled. Not even close.
How do I make an sRGB soft proof match a ProPhoto RGB print? And why would I want to?
This mostly depends on how well your viewing booth is set up to match your monitor when soft proofing. Seems that people have a lot of difficulty doing this. You have to understand illuminants, possibly create a profile targeting a specific illuminant if you aren't close enough to D50. Most aren't. Then get the illuminant WP, lux and monitor WP, nit levels correct. That's the hard part in soft proofing. Sure sRGB is a pretty limited space. Only use it if you have to.
Still, knowing how limited sRGB is, I'm just rather amazed how close almost all images I've looked at so far can be rendered in sRGB to proximate a high gamut print.
Also, someone might look into making 3D LUT monitor profiles that map like sRGB inside sRGB gamut but provide a better algorithm to the gamut boundry than RGB clipping.