gillymor:
"Can you emulate the kind of blues ornaments that are commonly used on stringed instruments like slides, bends and vibrato on the E.C?"

gillymor,
Taking your words literally it IS impossible when you speak of *stringed* instruments.
"Slides" may be simulated with the same trick as with pianos.
"Vibrato" ( in the meaning of a continuous variation of pitch which may be performed with a violin or guitar for example is in principle impossible.
"Bending" ( as the term is used with harmonica playing which also is a variation of pitch "can" be done with concertinas but not as effectively as with a harmonica because the concertina reeds do not offer the same means regarding forced control of the wind as the harmonica reeds do. On top of that when bending on a harmonica there are several techniques, some involving use of tongue and lips and thus directing the air flow in the compartment formed by the mouth and the mouth may be used also for resonance. You miss all that with the squeezeboxes of course

BUT....nevertheless you *may* manage some "bending" also with concertinas !, but it IS quite difficult, and fewer reeds admit it than with the harmonicas. When learning the procedure you have to consider the risk damaging a number of reeds in the process...
The only player I have met who really masters the trick is Bertram Levy ( with the Anglo however) but there may be others too of course.