EBarnacle:
before/above :"just do it. I have also played various forms of jazz and salsa. Go for it. It's just another skill"
Guran RE: A skill no doubt but as I said before I don't regard it as all that easy..

EB now: "Try slowing or speeding your bellows movement--crude but effective".
Guran RE: With the harmonica you may bend a half note down (or up) or even more, that is 100 cent or more. Just "slowing or speeding" a bit with the concertina bellows may "bend" = decrease the pitch for lower octave reeds or (possibly) increase the pitch for top range reeds about 10 cent or possibly 20 - but that is not even near as effective as on the harmonica. Keeping in mind that the "better" the concertina reed the more pitch stability it has, and the more difficult it is to get any effect at all. Harmonica players use to notice that low class harps often bend easier ( due to bad reeds, same of course with concertinas) but there are also top class blues harps that got reeds intentionally tempered to bend easily.
One way stressing the bending effect may be to manipulate the pad opening at the same time as the tone onset by staying with just a very slight lift for a short while. Do you practise that? Anyway - do you have any recorded sample to illustrate what you do?