Modify the .htaccess file in order to cache page headers

"Cache our page headers" probably isn't what you mean.

The caching HTTP headers (​Max-Age, ​ETag, If-None-Match, ​and related) on a main document don't apply to assets loaded within the document. Those assets need to have their own ​caching related headers: a main document with a far-future expiry won't stop its inner <script> and <link> assets from reloading from the origin, for example.

If granular control is a concern, you should serve your Marketo LPs + assets via your own CDN.  I wouldn't expect Marketo to allow you to tweak HTTPd-level settings (also note the .htaccess hidden​ file ​is just an Apache convention and isn't the only way to manage cache behavior).

You should also turn on this Treasure Chest feature:

And remember that personalized LP content, for reasons that should be obvious, cannot be cached.

In addition to the feature Sanford mentioned, we're actually rolling out even more robust nginx caching for all customers. I don't have a timeframe but shouldn't be too long before it's fully deployed.