Maybe you're overthinking this. Stress marks only place the stress, but indeed do not tell the speaker how to pronounce the word. In the case of tríada it's clear how to pronounce the word, and in the case of guion/guión you write or not the stress mark depending on how the word is pronounced in your region (as a 1-syllable word or a 2-syllable word). And that's the point: how the word is pronounced depends entirely on how it is pronounced in your region, and nothing more. You just need to follow the rules accordingly to your preferred pronounciation.
I think your question may be similar as asking how do you mark in the word zapato that the z must be pronounced as a s because you live in a region with seseo. You do not need to mark that in the word, just let people pronounce it the way they want.
Example: the word triaca. In the DPD it is stated that the word is stressed in the penultimate syllable, but it does not state which syllable is that, because for some people it will be tria and for other people it will be a. As long as you follow the rules, it is safe to just let the people pronounce the words the way they want.
Another different thing is that you may want to force people to separate the syllables in triaca because you need it for properly counting the syllables in a verse inside a poem. In that case you could use a dieresis, as stated in the Spanish Grammar:
Solo en la edición de ciertos textos poéticos, donde puede ser esencial a efectos métricos determinar con claridad el número de sílabas de que constan las palabras que aparecen en los versos, se señala en la escritura si una determinada secuencia vocálica [...] ha de ser considerada un hiato. Para ello se utiliza otro de los signos diacríticos del sistema gráfico del español, la diéresis, que se coloca en ese caso sobre la primera vocal de la secuencia: süave [su.á.be].
Only in the edition of certain poetic texts, where it may be essential for metric purposes to clearly determine the number of syllables that the words that appear in the verses consist of, is it indicated in writing whether a certain vowel sequence [...] needs to be considered a hiatus. For this, another of the diacritical signs of the Spanish graphic system is used, the dieresis, which is placed in this case on the first vowel of the sequence: süave [su.á.be].
So you could safely write in your poem trïaca to force people to pronounce the word as a 3-syllable one.