Can't be done, at least not easily.
But there is a pre Safari Reader solution that will probably make you happy. Instapaper has a reading mode very similar to Safari's Reader mode. Let's just say that Apple was apparently inspired by Instapaper and a few similar offerings.
Get an Instapaper account, and the app for your iPad.
Once you do that you can create a special bookmark in the BookMark Bar. When every you tap it the page will be saved to Instapaper.
To do that, you need a bit of javascipt code.
Then you make a bookmark for any site, and edit it, changing the name and pasting this code into the URL part. The following guide should make it easier.
When you copy the code on the iPad just tap and hold the code until it is selected, be sure to drag the selection handles to exclude the Code: header.