I look for published works on direct susceptibility testing especially in poultry. there are several works regarding human practice but I could not find works on animal side.
With direct antibiotic susceptibility testing I guess you mean inoculation of the sample directly on the MH plate. I would strongly advise against this for a variety of reasons, the most important being that the results' interpretation would be totally unreliable since contents in the sample may interfere with the test and you do not know the size of your inoculum' to mention just 2 of many reasons.
I, completely, agree with you. I have found in the litterature works about its use in human medecine in urinary tract infections but no study to have palpable (published) data about its usefulness and limitations based on results in veterinary medecine.
You can use CLSI guideline Vet 01-A4 (Susceptibility Tests for Bacteria Isolated From Animals.......). You can purchase it from CLSI bookshop online. However, a sample for this guideline is attached here.