Is anyone aware of any writers associated with "brethren" who interpret the baptism of Romans 6:3 as not being water baptism, but rather the believer's union with Christ at conversion (as John Owen, Martyn Lloyd-Jones, Donald Barnhouse, James Boice, etc. understand it)? 

Rodger, I can think of none, so far. BTW, John Gill in his exposition of the Bible, and who agrees with the water baptism and death correlation, and also alludes to the union with Christ, seems to give a balanced view on this verse; seldom found.

Thanks for the tip, Syd!