You can get pretty liberal with it. If it isn't accompanied by furigana, I'd read 大電車 as だいでんしゃ or おおでんしゃ; I guess you could read it as おおきでんしゃ, but usually おおき is written 大き (though I've read tons of literature that just have 大 already and still read as おおき, but it's pretty uncommon/irregular).
成当雨商品泳 can be read a shit ton of ways, and I be too lazy to list them. Japanese naming is pretty convoluted because parents usually try to name their kid something special, but not special as in the pronunciation of their name, but special in the characters used to form their names, that's why it's pretty fucking annoying to guess someone's name if only given kanji. It's also why when people introduce themselves via naming, they'll tell you the characters that make up their names as well. For example:
"MY NAME IS XXX XXX, WRITTEN WITH THE CHARACTER FOR SEX AND THE CHARACTER FOR BLOWAHUGELOADINME."
Or something like that.
EDIT: Oh right, forgot to mention there are also names that use no kanji at all (and it's not limited to only foreigners), but that has nothing to do with coming up with net names.