Warning: this post may cause you to skip a popular shabbos zemer -- don't say I didn't warn you.
This week I want to mention another chiddush of the GR"A that I don't fully understand. If you pay attention to the words, Tzur Mishelo is basically a song of thanks to Hashem for the food eaten at seudas shabbos. The stanzas "Hazan...," "Al kein Nodeh...," "Rachem...," "Yibaneh haMikdash..." parallel the brachos of birchas hamazon and echo the same themes. Therefore, as reported by R' Chaim Volozhiner, the GR"A avoided singing Tzur Mishelo lest it count as a fulfillment of his chiyuv d'oraysa of birchas hamazon.
I'm missing something here -- so what if it may count as a fulfillment of the chiyuv d'oraysa of bentching? Since one would not be yotzei on a derabbanan level by reciting Tzur Mishelo because it lacks the formula of shem and malchus, there should not be a problem of bracha l'vatalah in reciting the full bentching (and according to the Pri Megadim that we discussed here I would even venture to say there is not even a d'oraysa kiyum in singing Tzur Mishelo). Perhaps the concern is that if the zemer counts as bentching it means the meal has ended, which may not be convenient. But this too is a problem easily eliminated. One obvious solution is to sing the zemer at the close of the meal. Another solution: the Steipler (Orchos Rabeinu vol 1.) is reported to have made sure to eat a k'zayis of bread after singing Tzur Mishelo (so that his real bentching would be on a proper shiur achila) and also had in mind specifically that it is not a hefsek.
But there is a more basic problem I have with the whole chiddush. Mitzvos tzerichos kavanah, especially when speaking of a d'oraysa. I am willing to bet that a large number of people who sing Tzur Mishelo have no idea that it echoes birchas hamazon and have no intent for it to count as a kiyum mitzvah. The GR"A obviously did, but knowing what the words mean is not the same as having kavanah l'shem mitzvah. Just to be safe, you can even have negative kavanah while you sing, i.e. have in mind specifically to not be yotzei by singing. This seems to be even simpler than the Steipler's solution, which makes me wonder why the Steipler did what he did.