[bitxbitxbitcoin]

This would work for ñ but it wouldn’t necessarily work for replacing accents unless the vowel followed by u becomes an accent - which leads to the estadounidense problem you identified.

From what I remember from high school Spanish, there is a default syllable that has an “invisible accent” on its vowel in a Spanish word without accents and the purpose of an accent is to change the syllable that gets emphasized.

[schoen]

Yes, we'd ideally need to find a digraph that absolutely doesn't occur in Spanish. This can be tricky with compound words and loanwords. It seems that ou, oe and oo are super-rare in Spanish morphemes but can occur in loanwords and compounds. I just searched for unaccented digraphs that literally don't occur at all in /usr/share/dict/spanish and the only examples (of which there are 184 excluding k and w) contain only consonants and y.

So, there's not any easy natural way to do this without creating at least some ambiguities.