I wasn’t brought up in Reform Judaism, but the services of the Society of Jewish Science, which I attended for several years, were (to some extent) derived from the Reform service of the early 20th century. I once bought the Reform hymn-book, “The Union Hymnal,” too; I (rightly) thought it would be useful as a reference, even if I wasn’t singing the hymns themselves. I still have it, after 30 years.
In particular, the Jewish Science group sang four classical Reform Hymns in weekly rotation. We sang them without 4-part harmony (a missed opportunity, in retrospect). Congregational hymn-singing is now outdated (until it comes back into fashion, of course). Not much harmony these days, either. It’s a different era.
One of those hymns came to mind this week. I can’t say why: “G-d is in His Holy Temple” (no. 4 in my Union Hymnal). The author is “Anonymous” (what a shame). The composer is listed as “H.W. Hawkes.” There was a British Unitarian minister by that name; probably the same person. I’d have expected him to be listed as the writer rather than the composer. Perhaps he was both. Or, perhaps he was listed in error as the composer rather than the writer, while the actual composer’s name was lost (for now)?
In these beautiful words, we ourselves are G-d’s “Holy Temple” (see verses 3 and 4):
“G-d is in His Holy Temple,
Earthly thoughts be silent now,
While with reverence we assemble
and before His Presence bow.
He is with us now and ever
When we call upon His Name,
Aiding every good endeavor;
Guiding every upward aim.
G-d is in His Holy Temple,
In the pure and holy mind;
In the reverent heart and simple;
In the soul from sense refined.
Banish, then, each base emotion
Lift us up, O L-rd, to Thee,
Let our souls, in pure devotion,
Temples for Thy worship be.” 
The body as “the Temple of G-d” is a familiar Christian metaphor:
“…your body is the temple of the Holy Spirit in you…” 
But it’s found in Jewish sources, too; notably in the song “Bilvavi” by Rabbi S. Brazil,  popular especially in the “yeshivah world.” The metaphor also appears elsewhere in Jewish sources.
Is there a difference between “G-d is All” (as taught in Kabbalah and Hasidut) and “G-d is in all?” Only in this sense: “G-d is All,” whether we’re aware of it or not; the personal experience that confirms it might be expressed as “G-d is in all,” as when Ya’akov said, “G-d is in this place and I didn’t know it.”