I've been playing around with ID CS6 (OS X) for a while and I can't for the life of me get it to recognize a pasted LF as a forced line break. LF and CR and CRLF all go to paragraph breaks. U+2028 and U+2029 are display as empty glyphs, not breaks.
I'm a little wary of posting this as an answer, but I'll give it a go:
You might consider providing the text as a downloaded .txt file. CS5 introduced "Tagged Text" (a sort of XML-ish text document with full support for InDesign characters, attributes, etc.,) so this means your designers will be able to place the text file and InDesign will treat everything as intended.
To turn your existing text into CS5+'s Tagged Text (see the reference here), plop a <ASCII-MAC>
or <ASCII-WIN>
(as appropriate) as the first line and escape any '<' or '>'s with a backslash, then you're free to use <0x000A>
as a forced line break. (literally those 8 characters)
That's probably mega-overkill, but it's certainly the most stupidly reliable way I can think of. I'll edit if I get anything else working.
NB. "forced line break" is the term InDesign itself uses for the character produced by Shift+Enter, your "soft line break;" contrast with "paragraph break" for a standard carriage return. InDesign apparently represents forced breaks with LF (U+000A) and paragraph breaks with CR (U+000D).