Welcome to the forum Jens
I have listened to a lot of slide guitar, lap steel, dobro, pedal steel, and related instruments. Like you, I love the soul and microtonal fluidity of a bar or bottleneck sliding over steel strings, and have tried to emulate that effect on harmonica - especially with the AsiaBend.
In my opinion, there is no one 'sound' of slide guitar. Every instrument is different, and their sound varies a lot depending on whether it's acoustic or electric, what effects are used etc etc.
What I think you're really talking about is the seamless bending effect this class of stringed instruments utilises. They can bend every note, as little or as much as they wish.
If you want that, before going to MIDI, the first thing to look at is your harmonica. The standard Richter harp is very limited in this respect. Out of the 20 reeds, only 8 can be bent. Five can only bend one semitone, two can bend two semitones, and only only one can bend three semitones.
From my studies of various music genres where bending is used extensively, I found that a range of between one and three semitones form the vast majority of bent notes in actual practice. But you need bends on every note on your harmonica! However, not necessarily the full 3 semitones everywhere: you're playing within a key or mode, the harp can be tuned to give appropriate depth of bends, depending on the scale note.
But you definitely need an X-Reed harmonica to achieve this, and one with 4 reeds per chamber. The XB-40 is such a type, as is the AsiaBend.
So... That's the first area to attend to. After you have an all-bending harp, how does seamless note bending work with MidiGuitar2 in the iPad? So-so, in my experience...
The Midi sound doesn't track perfectly on bends, and tends to 'step bend' between the semitones. That being said, if your harp is highest in the mix, the Midi sound appears to be following it pretty well.
Hope that helps!