Should be no problem at all. After all the broadcasters mixed them for years. I have 4 analogue UHF channels mixed with Freeview and an HDMI modulator all fed to eight points around the house and it works well. Just get the levels and frequencies right and it should work.
A loft box is just a distribution and mixing centre, something you can makeup yourself with discreet components.
Using good quality transmitters you are correct, should be able to mix analogue and digital on the same cable, we have repeaters where the in and out are only 600 apart which I have used a lot.
However most of the RF outputs come from set top boxes, and it would seem these are poor quality, not only that clearly the TV's are not the best, in theory the analogue signal should not be degraded by the digital signal as the digital received on the aerial should be clean, but it is effected so the TV must have a poor rejection.
Theory as you say it should work, in practice however from bitter experience it does not, it could even be the cable is not quite the right length just 9" can make a huge difference, using a VSWR meter I have trimmed aerial leads in the past to improve signal, however the VSWR meter will not really measure the mW used, I was licensed for 400W although normally no more than 45W used, but the meters will not work with TV transmitters.
So although you can, often it is not worth the effort, having feeds from the LNB into each room, and using the RF from the Sky box as it may be viewing a encrypted channel is likely good enough. Even then it is likely you can watch Sky on the PC anyway.
I have redundant cables everywhere in my house where I have done things like have double telephone cables, one local fed from fax machine so once fax cuts in all telephones are isolated so the fax has a higher baud rate, at the time we had two numbers with different rings one would auto start fax machine. Non of this is now used. The last fax I sent or received must be 5 years ago, one relative still had a machine, when she died we stopped sending or receiving fax. It was after all a 19th century invention.
But in general we don't need the cables we did years ago, and modern units don't even have the option, can't plug my phone into the LAN it's wireless or nothing.