Just recently, a new repo working group has been established (https://www.fixtrading.org/groups/repowg/). Maybe you want to join that group to find out more about what they are/will be working on.
There are also extensive and excellent guidelines related to fixed income, posted on the FIX website (https://www.fixtrading.org/recommended-practicesguidelines/). I suggest to download the one called “Best Practices for Bonds”. These were written back in 2014 by the Global Fixed Income Committee and are still valid. You will find numerous workflows for quote-driven workflows, CLOB workflows and post-trade workflows.
Have a look at Volume 3, Chapter 3.4 Scenario QDM1: Quote Request – Dealer Quotes – Quote Accepted and Chapter 3.5 Scenario QDM2: Quote Request – Dealer Quotes – Quote Rejected
Also Chapter 6.10 Scenario QDM29: Indicative Quote – Customer Counters – Dealer Counters