First of all - thanks very much for all the detailed and considerate responses - much appreciated. Secondly, I guess I'm still feeling like I haven't quite explained my request clearly enough (no surprise there!). So when I say 'amp in the room' what I mean is 'playing in the same room as the amp'. Something happens in this situation between speaker and guitar (and yes depending on many variables such as position, volume, the room etc etc) that changes the way the sound behaves, particularly high gain sounds, and with higher notes up the fretboard. Exactly the same issue arises when recording a real amp or speaker cab that is closed off in another room (if the guitar is in the control room and the amp/cab is in the live room for example).
Specifically what I am after is a way of simulating the boost to higher frequencies (notes of the upper strings and especially when higher up the fretboard) that occurs when the player is in the same space as their amp. With high gain sounds sustain is often much longer, a lower gain setting is required to create a usable lead tone particularly when playing legato and so on. With a modeller (or remote cab) I find myself picking and fretting much more aggressively to achieve the hoped for tone the higher up the neck I go. Which then catches you out if you happen to use the sound live in a higher volume situation. All of this is also way more noticeable with single coil pickups of course. Compensating in the modeller by turning up the gain helps, but leads to a 'fizzier' sound of course. I'm not aware that speaker simulation seeks to emulate this 'boost/resonance' but rather to tame higher frequencies to make it sound less digital - and this quite often has the opposite effect as in it reduces treble like a close mic'd speaker would.
I'm hoping a modeller would be able to recreate this idea at any volume, that is a playback system is not even required to create it. And I'm not after a faithful recreation if that doesn't exist, simply tips for compensating if anyone else has successfully addressed this.
If you feel I am simply repeating myself and missing the point (again!) then please accept my apologies and ignore, but just posting in case it catches anyone else's eye. Thanks :)