Is it dry in Winter? or just in Summer? I'm thinking probably only in Summer?
Would you consider installing a Fogging jet? They are only a few quid, although if you wanted it hooked up to a timer / humidity sensor that would jack the price up, but if you are around morning / evening it might be enough just to turn it on for a few minutes. You could spray the plants with water morning and evening. If you have hard water that will probably leave water marks on the leaves, which may make them look unsightly; may also cause sunburn (light magnified through the water droplets, although some people think that is a myth I reckon it happens in practice)
The traditional way to raise humidity in glasshouses in Summer is to "damp down". Flood the floor with water, and let it evaporate. Added benefit that the latent heat of evaporation means that the process absorbs/consumes heat, so it reduces the temperature too.
An alternative is to stand the plants on gavel trays, or saucers. Put an inch, or more, of gravel in the tray and sit the pot on that. Fill up the tray with water (but not so much that the pot base is sitting in water), and then theoretically the evaporating water rises up amongst the leaves of the plant above. I've always been sceptical how much difference this actually makes, but assuming it works it may be kinder to the structure of the conservatory than raising the humidity in the whole structure.
Sorry, not answered your question, but personally I would prefer to choose plants that I would like to enjoy, and then provide the conditions they want (assuming not impossible / horrendous to achieve).
My ultimate Conservatory Plant would be Strongylodon macrobotrys
but it requires incredibly high humidity levels, so is out of the question really
Mucuna bennettii might be a suitable alternative:
My shortlist of highly desirable conservatory plants includes Lapageria, Plumbago capensis, Strelitzia, some passion flowers - lots of unusual ones available. Some unusual bulbs to ring-the-changes with the seasons, such as Hymenocallis. Climbers up other, bigger, plants such as Aristolochia perhaps. Maybe even some really tall Fuchsias - something that can be trained over a few years and provide a spectacular display once full size.
Orchids (e.g. Cymbidiums) for the Winter perhaps? They can go outside during the summer (they need Day/Night temperature to fluctuate during Summer to create flower buds for the Winter). They need 11C night and 21C day in Winter ... so would perhaps need to move in/out of Conservatory accordingly. Too much hassle perhaps.
I'm growing a Travellers Palm (from seed) for mine. Bananas might be OK too (fine in Summer, look a bit manky in Winter with low light levels and 5C temperatures)
Heliconia rostrata
Thunbergia mysorensis
Kennedia nigricans
Bat plants (Tacca Chantrieria) maybe?
Solandra maxima / grandiflora