'He braced his foot in the stirrup' is just fine.
He made sure his foot was firmly and safely positioned in the stirrup before mounting his horse.
If you need a dictionary reference to convince you, see
(Collins)to steady or prepare (oneself or something)
The dictionary link you provided also gave three examples of 'braced for', so 'against' is not the only preposition that works after 'braced'.
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