Not a lot of space left for you on this one.
I'd just do as you already have - grab some roots and change with the chords. There was so much going on I had trouble hearing the chord changes. Grab a basic beat and hang on is about all I would be able to do with this.
That does not help your question about what bass line to come up with.
I hear the tonal center as being the key of D - that being the case.......
I, V, viidim, IV
D,A, C.......G ........ see if C#dim fits better
You already mentioned all the normal choices to use with a progression like this.
Over the D chord as it looks like a basic major triad do a R-3-5-3 or R-3-5-8.
Over the A chord same thing so do another R-3-5-3
Over the C#dim do a R-b3-b5-bb7 if you have the space for it. If not go R-b3-b5-b3. If the other quys are using C#dim then the b3 and b5 are important. If they are going with C then it does not matter, another R-3-5-3 will work.
Over the G chord as it is another basic major triad go back to that R-3-5-3
Or --- you basic progression revolves around the D, G and A major chords and R-3-5-3 works with all three of them. I go back to there is not a lot of space left for you beyond that repeated root and it does work fine. I doubt anyone could hear the difference in that C or C#dim anyway.
This can help with what riff to use over the chords.
http://www.looknohands.com/chordhous.../index_rb.html
C# dim7
intervals: 1,b3,b5,bb7
notes: C#,E,G,A#