Clematis alpina and varieties are good, scambling clematis for shrubs and are not too vigorous (unlike the montanas which are thugs for this sort of use). These are spring/summer flowering so you should be looking at shrubs that are not particularly grown for their flowers or are early flowering like medium sized Rhodedendrons.

I grew many clematis through shrubs at my former house/garden.of course it depends on the size of the shrub/tree and how strong a grower it is.but, I, very successfully grew Niobe(a fairly large flowered variety) through Hypericum(not the short spreading type but the taller, mound forming shrub. The reddish/purple flowers looked superb with the yellow hypericum blooms. I also grew Warsaw Nike through a climbing 'Golden Showers' Rose. Some herbaceous Clematis make a good show growing through bigger shrubby plants also.

Thanks Bulb, Paul. I'm getting the idea now and thank you for your suggested combinations.