I imagine this question's been asked a few times but in the hope of progress..

Has anyone created custom songs for 5 String Bass?

 

Gutted I can't use the low B in game. Songs from artists like Jamiroquai would benefit in a big way!

 

Has anyone managed to trick the game into showing 5 strings on the note highway?

I'm sure someone in this talented community has worked something out?!

I think nest time you should at least search for it before asking ;)

 

http://search.customsforge.com/?q=ii_5stringbass

I will add this, whether or not a note lower than C works depends on if the tracker has added the low tuning fix. So some might work, and some might not.

firekorn has alot of nice 5 string bass tracks (I'm also an In Flames junkie though...)

As for the issues with low B tuning, checkout this addition I made to the RS Toolkit. It may help you. There's still a few bugs with it that I plan to fix, but I haven't had the time as of recent. 

http://customsforge.com/topic/10916-new-low-bass-tuning-fix-for-rs-toolkit/

I have two FFDP , Hard to See and Bad Company in B Tuning, it's 4 string because I didn't have to use the hi-string, 

works fine on my 5 string bass. There is also a discussion of the way to achieve B-Tuning, but recognition is not so good with emulated bass unless you have thick strings.

Many of the In Flames track don't use the highest strings but i do this because the bassist use a 5 strings bass and that it sometimes changes the way the bass part need to be tabbed in the first place.