based on what you say, i think your best option is to create epub files. i imagine you have see
this thread already, since you've figured out how to use external fonts.
it is true that currently the 700 does not display justified text ; however, this is actually due to a lack of support in Adobe's epub renderer, not Sony's fault. This shortcoming has since been fixed (the latest version of Adobe Digital Editions does display justified text), so i think it's not at all unreasonable to think that a future firmware update from sony will integrate the latest version of DE and correct that. I know a lot of people would like to have justified text in epub (personally, i've got nothing against left-aligned text ; i actually prefer that to a badly implemented automatic justification, as i can't stand to see "holes" and "rivers" in a page of justified text, which happens a lot if you use medium size display, as i do, but i think i'm in the minority).
i know, this is an imperfect solution or compromise, but the epub format is still being developped and improved, and will only continue to improve, as will the rendering engines.