I would ask the Pastor in advance. I’d also say that it was to remember the great honor of this day (and not an autograph).
My cousin, a priest, has a missal with the names of every wedding and baptism he has performed over his 38 years. This is not a frivolous request; even priests (and Bishops) take great pleasure in this and look back on these things fondly.
In that he’s asking his sponsor, his aunt, to also sign, of course the request really isn’t in the nature of seeking autographs, but, as you suggest, to remember the great honor of the day.
Thanks.
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