Sounds a bit Japanese: "夕食の準備は", assuming that Japanese "-wa" is Toki Pona "la". Since "la" is introduced in Pu as a "lon" in disguise, double-checking this structure as "pali ona li kama pini lon moku pi tenpo pimeja" or "pali pi moku pi tenpo pimeja li kama pini lon ona" yields problematic sentences. For ma pona people this sentence is still readable, I guess, so I just give it the "ma pona" tag.

"in the context of dinner, it's preparation is finished" seems defensible to me, though yeah, you see right through me: it's a contortion to avoid multiple "pi"s in a single noun phrase... ^_^

"ma pona"-ing makes sense though - they do "la" a casual way that raises eyebrows elsewhere.

[ Getting in grammar-lawyer mode: The statement that lon sentences can be rewritten as la statements does not assert the converse, so while it could be used to validate certain sentences, it doesn't definitively disqualify them. Though I had a look and can't find any relevant sentences in Pu where this doesn't work as a test. ]

In Pu, Sonja tries to be as backward-compatible and easy-to-read as possible, yielding formulas like "if A, B" or "in the context of A, B". (Oh, I remember we have talked about that already - of course.) Does that mean that whenever I can say in English "in the context of A, B", I can say in Toki Pona "A la B"? Many defend their "la"s this way, so de facto it is "defendable" this way, yes.

To me, there's a difference between "in the context of the party, I danced" and "in the context of the party, I liked it". The latter is short for "speaking in the context of the party, I liked it", but I don't see how this abbreviation is happening in Toki Pona:

"kulupu musi la mi tawa musi." OK
"kulupu musi la ona li pona tawa mi." CALQUE

"in the context of" is such an elastic expression in English, that you can use it for everything - in English. And honestly, between you and me, I dislike it when people (I don't mean you) say "context" and actually mean "I can't be bothered to take this issue seriously from a grammatical point of view - i.e. without simply calquing from English if I have to".

Applied to the sentence at hand: "in the context of dinner, its preparation is finished", in my eyes, is short for "speaking in the context of dinner, its preparation is finished", which is short for "while we're speaking in the context of dinner, its preparation is finished", which is "mi toki e ijo pi moku pi tenpo pimeja, la pali ona li pini." I wouldn't shorten it to "moku pi tenpo pimeja la pali ona li pini." without having a grammar book or a course or something to back it up. ma pona is its own institution, so to speak, so "ma pona" does what ma pona does. :)