A word for “beyond” in the Namárië poem as well as the 1960s version of Markirya poem, in both cases uses as a post-position in the phrases Andúnë pella “beyond the West” (LotR/377) and elenillor pella “[from] beyond the stars” (MC/222) respectively. It was also used in the second-to-last version of the Átaremma prayer from the 1950s, appearing as a preposition in the phrase Átaremma i ëa pell’ Ëa “*our father who is beyond existence” with pell’ revised to han in the final version of the prayer (VT43/12).

Tolkien specified that the more precise meaning of pella was “beyond the borders of” (RGEO/58) and that it was “only used of structures, natural or artificial, forming a fence or wall or a boundary” (PE17/92).

Possible Etymology: In Words, Phrases and Passages from the Lord of the Rings (WPP) from the late 1950s or early 1960s, Tolkien derived pella from ✶peth-la (PE17/64). In notes from around 1965, Tolkien tried to explain why pella was used as a post-position. He first considered making its second element lá a deictic (demonstrative) element meaning “far away”, and made its initial element be √PEL “edge, bound, fence, limit”, so meaning something like “*far away there past the boundary”. But he became dissatisfied with this derivation due to similarities with French là “there” (PE17/65).

He then decided that lá was an ordinary preposition meaning “across, over, beyond”, and the initial element should be √PAL as seen in palan “far”, resulting in a new form palla, saying that its use as a post-position in Namárië was poetical rather that normal grammar (PE17/65). However, this new form palla was not applied in the second edition of The Lord of the Rings, and in the “prose” version of Namárië from The Road Goes Ever On published in 1967 pella was still used as a post-position (RGEO/58).

Finally, in notes from 1967, Tolkien said that the original formation was Andúne pelo lá as in “beyond the West fence”, which was then reduced to Andúne pella and reinterpreted as “beyond the boundary of the West” (PE17/92).

Neo-Quenya: For purposes of Neo-Quenya, I would assume pella is a preposition or adverb meaning “beyond the boundary of”, useable only of things that form boundaries like mountain ranges, fences, and rivers. For “beyond” in a more general sense, use Q. lá. I would further assume that due to quirks of its origin (pelo lá > pella) it tends to be used as a post-position, but can be used as a preposition as well.