Have P-acyl phosphines ever been synthesized?
When I saw this question, I realized I couldn't remember ever seeing an acylphosphine, but I was curious and went looking. I found that they're actually quite easy to make. One common thing I saw was the use of trimethylsilylphosphines with acyl chlorides to form acylphosphines.

The easier method, however, was simply to react a substituted phosphine in the presence of triethylamine with an acyl chloride. The analogous reaction can be done with amines to form amides. In this paper, they describe the synthesis of P-acetyldiphenylphosphine using diphenylphosphine, triethylamine and acetyl chloride. They have some other examples in there, but this suggests that these compounds are typically fairly trivial to synthesize.

One more note: In my searches, I saw "acylphosphine," "acyl phosphide," and "phosphomide" as named. The paper I linked uses the term "phosphomide."