When is asking for an update on PhD admission status appropriate (given international flight prices) I am a US student who has applied to physics PhD programs while aboard on a Fulbright Scholarship in the Netherlands.

Based on the dates of open houses last year, I will be heading back to the US for the week of March 14-21 to attend graduate open houses.

Physics PhD programs allot roughly 500 dollars per admitted student for travel expenses for them to attend the open house. This was totally sufficient last year when I attended open houses as I was in the US.

Hypothetically, if I get into Brandeis on February 20th, buying a plane ticket to Boston from Amsterdam 3 weeks in advance would be doable — roughly $350.

However, hypothetically even if I got into USC later today, a plane ticket to Los Angeles would still mean money out of my pocket.

If I submitted my applications to December 15th deadline programs around December 5th-10th, is reaching out for an update around February 1st too pushy?
The general rule for you is that you can ask. It would be good to explain in any request that you need to arrange international travel and that finances are an issue. 

But the general rule for the university is that they may be very constrained in what they can tell you. There may be rules or agreements about deadlines and about giving out information early, even if it is available. So, you may not get much of an answer and it would be very unlikely to get an early decision. 

Graduate open houses are useful, but perhaps not necessary in every case. Make reasonable plans but stay flexible and expect that things can change. Money out of pocket probably isn't your most important consideration.