Rav Shmuel Baruch Ganot, a rebbi and posek, says that he was asked by several congregants, or gabbaim, of a shul that publicized their scheduled service of Hallel with musical accompaniment in a shul and several avreichim expressed protest of this type of a service claiming it is changing form tradition, if it is allowed or not.

Rav Ganot says that poskim in the past have dealt with this issue already and generally have paskened that it is not allowed because it both looks too similar to the service of the Reform, and also because it is inappropriate to do so when standing in front of the King. Rav Ganot says there might be a difference between the regular part of the davening and Hallel, as even the Levites in the Mikdash sang Hallel and played instruments along with it, so perhaps during Hallel it is appropriate even when standing before the King. Additionally, perhaps in Eretz Yisrael there is no problem of being similar to the Reform and there is no concern we might get swept up and follow in their ways. Presumably this is because they have so little influence or impact here in Israel.

Rav Ganot points out that the shul of Rav Dov Kook, the mekubal in Tiberias, sings Hallel with musical accompaniment, but they do so with seriousness and a sense of exaltedness, so the style is different than the style of the services being discussed.

Rav Ganot concluded paskening that in the shul they should hang a sign letting everyone know that Hallel will be sung with musical accompaniment with a serious attitude and atmosphere, and not in any way with letzanus and fun. Additionally the sign should say this is a one time event as it was already announced but it needs to be discussed further with the gedolim before such a service will be planned again.

Rav Ganot expounded further on the matter with no definitive conclusion and says he asked Rav Chai Kanievsky about Hallel with musical accompaniment and Rav Kanievsky said it is not appropriate.