Pruning group 1 are all species clematis and purple is not really a color option with these. You can get some nice bluey/lavenders with the alpinas or macropetalas however. I would recommend a pruning group 3 vine. These prefer to be hard pruned annually, which I find easiest to deal with when growing through or on other plants. 'Jackmanii', 'Gypsy Queen' or 'Etoile Violette' are all great purple flowered varieties.

Try and avoid watering on any foliage. It promotes all manner of diseases, especially with roses. You may have manually adjust your sprinkler head to accomplish this.

Viola is another great deep purple group 3. Venosa Violacea has a lovely purple and white blossom.

Thanks so much, both of you for your input! I'm going to take your advice and go with one from group 3. I'm trying to decide between 'Jackmanii' or 'Etoile Violette'. Both are very pretty from pictures I've found online. The sharpness of 'Jackmanii' flowers might contrast really nicely with the blousy blooms of MAC. Also, from info I've read it seems that 'Jackmanii' has a longer bloom period. Is that your experience?
Gardengal48: you're right, I should probably plan on trying to move or adjust that sprinkler head.
Thanks again!