I've sometimes cited Michael Kruger's work in support of the conclusion that 1 Timothy 5:18 is citing Luke's gospel as scripture. In a book published late last year, he provides a lengthier case for that conclusion. It's on pages 680-700 of Lois K. Fuller Dow, et al., edd., The Language And Literature Of The New Testament (Leiden, The Netherlands: Brill, 2017). If you don't want to pay the large price for the book, you can get it at a library or through interlibrary loan, as I did. Kruger's chapter is well worth getting and reading. He goes into a lot of detail, but here are some highlights:
Given that we have an exact match with a known source (Luke 10:7) - and exact matches are quite rare when tracing the words of Jesus during this time period - this raises the question of why we would prefer an unknown source to a known one. And there is another advantage of preferring the known source, namely that we know that (at some point) Luke actually acquired the scriptural status that 1 Tim 5:18 requires, whereas we have no evidence that any sayings source ever acquired such a scriptural status….
If it is too early for Luke to be regarded as Scripture, why is it not also too early for a written sayings source [a hypothetical alternative to Luke] to be regarded as Scripture? After all, one might think that Luke's purported apostolic connections (Luke 1:1-4) might allow his Gospel to be regarded as Scripture even more quickly than an anonymous sayings source….
He [John Meier, a New Testament scholar who's not a conservative and rejects Pauline authorship of 1 Timothy] states, "The only interpretation that avoids contorted intellectual acrobatics or special pleading is the plain, obvious one. [1 Timothy] is citing Luke's Gospel alongside Deuteronomy as normative Scripture for the ordering of the church's ministry." (689-91)