Rework for specific sizes.
In general, if anything needs to work in varying proportions. I feel, the best solution is to actually rework the piece to match the given proportions.
Marketing firms or higher-end companies do not try and use a single image for everything. They create multiple images based upon need. This is really the only way you can ensure branding and user impressions remain consistent.
Just as you would not try and force a "one page" or "flyer" to work on a billboard by merely resizing, you should not try and force the same advertisement to work for every conceivable outlet.
Create multiple images using the same content.
The minor amount of time necessary to rework the content will be directly reflected in the ROI of a better formatted image for a given outlet.
Moved from comments to clarify...
If reworking required you to edit everything 5 or 6 more times, you edit every bit 5 or 6 times. That's how it's done. Anything short of that is less than ideal. Typically, if someone were paying a marketing firm to advertise for them, they would be displeased with one image stretched and containing 50% white space rather than being designed to fit the advertising space allotted. Professionals rework the content to fit the space.
The point would be as a paying advertiser I've shelled out my money for a particular advertising space. It costs me money. A designer looking for shortcuts to save them work and delivering artwork that is only utilizing 50-60% of the space I am paying for, is not doing me any service. I'd look for a designer which understands advertising costs me money and I don't want to pay for 40% blank background merely because the designer didn't want to do a little extra work.
In addition, as the designer, you charge the client for the time needed to rework the content to fit various advertisers. I wouldn't do it for free. There would be an initial design fee then an additional $X amount for each different advertiser or size requiring a rework. Using random figures, that would be something like $300 for the design, then $50 per size/advertiser.