Minhagim: Answering Amen to Harachaman

In this coming week's learning quota from Laws of Daily Living, I noticed that there is a discussion regarding saying "Amen" to berachos that do not contain Hashem's Name. In addition, it mentions that one should answer "Amen" to each Harachaman in bentching.
I just wanted to point out that the minhag in Hornosteipel, as heard in the name of Rav Shoime of Denver, ztz"l, was indeed to answer "Amen" after each Harachaman (brought in Malchus Shlomo). Interestingly, the Radviller siddur (Tefillah Yesharah) also says that it is proper to do so (see below).

After a bit of research I found that although this is a fairly uncommon practice, it is clearly brought both in the Mishnah Berurah (189:5) and the Shulchan Aruch HaRav (189:6). The source is apparently a Kol Bo in the name of the Geonim, and the Shlah Hakadosh mentions it as well. The Magen Avraham (216:3), pointing to a Medrash, is also an advocate of this minhag.

Comments, anybody?