Hey David,
You've gotten great responses so far. Keep in mind though, there is a difference between note sustain and feeding back in key like what Mike has presented. There are some guitar players like Bats and Mike who like what we call natural sustain where the notes are sustaining from volume or just the right set-up for a player and the guitar. This relies more on right place, right time as well as making things loud enough to carry over. This is usually achieved by using less gain at the gain stage and either allowing a tube power amp to do the work, or extreme volume push through the right speaker cab to help...or a little of both at the same time. It's a great method that will give you a cleaner gain sound that won't go to mud. Guys in the 60's and 70's used this method and some of the great players of today still use it. It's what we like to call "pure tone" because it's not synthetically created using a gain stage.
The other method is to pump up the gain stage. The more gain you use, the more sustain you get. However, there are pro's and cons to this. With more gain comes a muddier tone unless you can compress it just right so that it tightens up the extreme gain and stops it from soundinglikearunonsentence of mud. LOL! Newer amps of today come with a much better pre-amp in them to where it becomes subjective as to whether you would want more gain at a low volume, or less gain at a loud volume for the purist type tone. Both work incredibly well.
But for what you are looking for, if I'm reading correctly, you pretty much want a guitar note to scream and sound like you pressed a key on the keyboard, correct? Like an infinitely sustained note, right?
This is hard to get. The only way I know how to do it is to use a Fernandes Sustainer (which is built into the guitar...Neal Schon from Journey uses one and he can hit one note and it lasts forever) or a Sustainiac system. All the other methods I use are hybrids of what Mike and Bats shared or you must have a really hot pre-amp (guitar pre-amp that is) to give you the right drive.
Now, with that TH2 and other guitar sims, most times you get loads of gain, but little sustain because amp sims are missing that buffered signal you get in a regular amp. You can put a compressor in your chain BEFORE the amp sim to sort of simulate this or even a light distortion pedal or something. This way the sound will react and sustain more like a real amp would. But most sims are lacking that sustain unless you can crank up your monitors and get a little power from the volume so things feed back like what Mike presented.
The only other thing I can suggest (which I've done often) is to get the hottest and most driven sound you can out of a guitar note, and then sample it. Once it's sampled, trim it up the way you want it and search the sample for even sustain...then copy that part and ammend it to the original by copying and pasting. Crossfade/edit so you can't hear where the ammended file was added, and you can make one note scream for minutes if need be. I've done this several times with success. Sometimes a player would hit a note and it would die out. He'd try to punch it in and do it over and over with volume, more gain etc...sometimes, a particular guitar and sound just doesn't equal "infinite". So you have to manufacture it.
I just would take some of the clip that sustained evenly, copy it and then paste it into a section to make it longer while experimenting with the slip-editing/cross fading stuff until it sounded like one big long note. That's the only thing I can really suggest that would work for you though. The TH2 has some cool stuff in it to bring up the input gain (from what I've read about it) so it very well may give you that simulated input gain most amp sims are lacking. I've not tried it for myself, but one of my friends is a tester for that company and he told me how awesome it was.
Anyway, hope some of this helps. Good luck. :)
-Danny
P.S. +1 on the Ebow...that thing is madness!