As a new member to the group, I just found this discussion so pardon the tardy reply! The way I understand modes may be a bit unorthodox, but I think I can help nonetheless. First off, the modes on guitar and modes on mando are the same, because the scale modes are theoretical concepts that are independent of the instrument. Thus, so long as you understand the underlying theoretical concept, all you need to do is apply that concept to the instrument of choice (easier said than done, right!). In the case of the mando, let's say you have a basic I, IV, V chord progression in the key of C. When soloing you could just hang out in the C major scale and solo to your heart's content. However, introducing some modal soloing should spice up the solo and provide and strengthen the relationship between each solo phrase and the chord under which it's played.
First off, let's describe modes a bit first. A mode is a variation of a scale that simply changes the tonic note. The actual scale degrees do NOT change...only the note that you start/end on. So, taking C major for example, the first mode of C major is called "Ionian" and this is the one that we all learn when we learn scales. We're just never simultaneously taught that its modal name is "Ionian." The notes are as follows:
C D E F G A B C
If we were to take this same scale, but start and end on D instead of C, the scale would be called "D Dorian." The notes would remain the same...it's just that our start and end note would be D instead of C.
So, in our I, IV, V progression, during the I chord (C), you'd solo using the C major scale in "Ionian" mode ensuring to start and end you solo phrase on C. Then when the IV chord (F) starts, keep the same fingering pattern for the C major scale, but start and end on F instead of C (this is called the "F Lydian" scale). Finally, when the V chord (G) starts, play the same C major scale pattern, but start and end on a G note (aka the "G Mixolydian" scale). The same logic applies for the minor chords as well. Let's say you have a ii chord (Dm) thrown in. What would you do? You guessed it! Still using the same C major scale pattern, just start and end on a D note (aka the "D Dorian" mode).
Modal soloing is dependent on the soloist's ability to hear the chord changes and adjust his/her solo accordingly. I think utilizing the same major scale pattern that corresponds with the key of the song is the easiest way to get started with this. Once you're comfortable with this, you could then explore differing patterns that correspond with the modes in different places on the fretboard.
In the case of the mandolin, take your closed position major scale pattern and memorize where the tonic notes are located for the chords in any given key. Start small and work your way up. For example, just start with the major chords. So, in C, memorize where the C, F, and G notes live within the C major scale on the fretboard. These locations within the pattern will be the same no matter what major key you're in. In this way, you can focus on utilizing a single scale pattern which can be moved up and down the neck depending on the key of the song. Once you're comfortable with soloing around the major chords (the I, IV, and V), start memorizing where the tonics for the minor chords are located (ii, iii, vi). If at any point you get lost within the solo, or forget which note you're suppose to start/end on, no worries. Just use the key tonic (C in our example) as home-base and it'll sound good.
I hope this helps and makes sense. Sorry if I explained concepts for which you're already familiar! Let me know if I can provide additional clarity on any of this.