Is a model-specific rainfly available? ...
Is a model-specific rainfly available? Is a larger size hip belt that is Deva-specific available and interchangeable? (say, a medium or large)
Model Specific, no. Gregory makes a general pack cover though, which can be found here: http://www.backcountry.com/gregory-rain-cover
I would suggest a small size, or a medium if you think you might be attaching things to the outside of your pack.
Any other general pack cover from another company, like sea-to-summit, should also work just as well: http://www.backcountry.com/sea-to-summit-ultra-light-siliconized-pack-cover
I would suggest a medium for that brand.
The hip belt should be sized appropriately with the pack size that you buy, but if you find that it is not you can buy a differently sized hip belt. Backcountry does not seem to have any in stock, but REI sometimes does, though I do not believe they are model specific. If you contact Gregory directly you may also be able to purchase one from them, though the don't have them listed on their website.