Multi-band audio delay filter?
Not sure about audio plugins but what you want to do is certaininly doable by using filters (delay line in your case in the subbands. Split the band using using a Perfect Reconstruction Analysis filter bank. Now plugin the delay line and then combine two subbands using synthesis filter bank. Note that you need to be extra careful while plugging in the filter between analysis and synthesis filter bank. You can check some of these papers for ideas on how to do subband filtering.

https://dl.acm.org/citation.cfm?id=2817121

http://ieeexplore.ieee.org/abstract/document/1661283/