Strictly speaking, no.
Museum visitor group in a city is usually quite small; nowadays "fluid populations" like tourists mostly rely on the Internet for deals and events, their chance of reading a local newspaper is close to none. This means your "targets" are not equally comparable at the beginning of every month when you try a new ad combo. Each month, you're actually starting with a more hard-to-reach group, or less-interested-in-museum group. Putting them into the same regression is not going to cause any mayhem, just that you'd need to interpret them very carefully, particularly with the sequence of your ad combos.
For this reason, I'd suggest why not stratify by month? Implement some tricks such as i) bring this ad to the admission for a gift/discount, ii) text this code or mention this code during online ticket purchase (which can be different on each type of ad, and on each wave so that you can account for lag effect) to get a discount, etc. After each month, count the ads or ask for the online purchase record from your IT person for analysis. That way at least you can say something about the effect between different ads, month by month. And you can be more certain what brought them to your museum.
I'd also recommend you to communicate with the publishers that their circulation has not changed in the test period, as it's not controlled in the model. A small ad may suddenly outdo a prominent ad just because it has increased some distribution points. You should also keep a sample of their publications to make sure it's not confounded by other major promotion campaigns which may have boosted the readership temporarily.
Same goes for your museum. Make sure there isn't any rotation in the theme exhibits. And beware of the combos happening across major holidays and summer vacations, etc.
Finally, any advertisement department of any reasonably big newspaper publisher would have conducted customer profile survey and compiled penetration statistics. If you buy ad from them, ask for that information. That can help you narrow down your selection to some more demographically aligned publications. And document all the costs as well.
I think my concluding statement is that it's a very interesting question, but regression may not be the right tool for this purpose, at least at this stage.