My next step is to explore the BHL Bibliography to find what information may be available to me about St. Agnes there. Her Latinized name is simply Agnes, which is useful. When I search for Agnes, four different saints appear, but my Agnes, Agnes v. m. Romae is among them. And she has quite a few sources available!

Bibliography
BHL 0156 — Passio auct. Ps.-Ambrosio
“Inc.: Servus Christi Ambrosius virginibus sacris. Diem festum ss. virginis celebremus…“
BLH 0156a — Passio auct. Ps.-Ambrosio
“Inc.: Servus Christi Ambrosius virginibus sacris. Diem festum ss. virginis celebremus…“
BHL 0156b — Passio auct. Ps.-Ambrosio
“Inc.: Servus Christi Ambrosius virginibus sacris. Diem festum ss. virginis celebremus…“
BHL 0157 — Passio auct. Ps.-Ambrosio (Recenssio brevior)
“Inc.: Servus Christi Ambrosius ad aedificationem virginum qualiter passa sit Agnes…“
BHL 0158 — Sermo (Ps.-Ambrosio vel Maximo ep. Taurin. adscriptus)
“Inc.: Cum in toto mundo virgineus flos Mariae…“
BHL 0158a — Sermo (Ps.-Ambrosio vel Maximo ep. Taurin. adscriptus)
“Inc.: Cum in toto mundo virgineus flos Mariae…“
BHL 0158c — Sermo (Ps.-Ambrosio vel Maximo ep. Taurin. adscriptus)
“Inc.: Immaculatus Dominus immaculatam sibi famulam…“
BHL 0159 — Carmen auct. Prudentio
“Inc.: Agnes sepulchrum est Romulea in domo…“
BHL 0160 — Carmen auct. Adhelmo
“Inc.: Floruit in mundo quaedam virguncula…“
BHL 0161 — Passio metrica
“Inc. prol.: Iam vulgata licet maneant exempla salutis — Inc.: Tertia post decimam fieret cum virginis aetas”
BHL 0162 — Carmen auct. Flodoardo
“Inc.: Inclita nobilitat venerandae virginis…”
BHL 0163 — Passio metrica auct. Hrotsuita
“Inc.: Virgo quae vanas mundi pompas…“
BHL 0164 — Passio metrica auct. Petro Riga (olim adscripta Hildeberto Cenoman. vel Philippo Harvengio)
“Inc.: Agnes sacra sui pennam scriptoris inauret…“
BHL 0164b — Passio metrica
“Inc.: Virgae regalis de germine flos specialis…“
BHL 0164d — Dialogus metricus auct. Gutolfo mon. Sancrucensi
“Inc. prol.: Agna sequens agnum — Inc.: Quid modo cunctamur?”
BHL 0164e — Carmen de Passione
“Inc.: Agnes sponsa Dei virgo ss., salve…“
BHL 0164f — Passio
“Inc.: Expectat desiderio mens diu pressa scelere…”
BHL 0164m — Miraculum Romanum anno 817
“Inc. prol.: Ad honorem Iesu Christi, qui in sanctis suis — Inc.: Erat tunc temporis monasterium…“
BHL 0165 — Translatio (Agnetis et Benigni) Ultraiuectum anno 964
“Inc.: Tempore quo fervente Danorum crudelitate…“
BHL 0165b — Epitome translationis (Agnetis et Benigni) Ultraiectum anno 964
“Inc.: Tempore quo fervente Danorum crudelitate…“
BHL 0166 — Miracula (Agnetis et Burgundofarae) Turonibus anno 1141
“Inc. prol.: Universis s. ecclesiae praelatis… Rarus et iam nostris temporibus — Inc.: Farensis monasterii sanctimoniales Farae virginis corpus, martyris Agnetis caput”
BHL 0167 — Miracula in coenobio S. Agnetis apud Swollam, auct. Thoma Kempensi
“Inc.: Frater quidam domus nostrae capitis dolorem…“
BHL 0167c — Miracula
“Inc.: Igitur dum parentes b. Agnetis…“
English Translations
Finding full English translations is harder than I thought it would be. Perhaps I am not looking in the right places, or perhaps I am looking through the incorrect manner. Perhaps the problem is similar to the one I ran into with earlier stages — St. Agnes is so common and popular that useful sources get buried underneath the Bible studies, devotional websites, and encyclopedia entries.
The most frequent response I can find when searching for any of these texts is analyses and summaries of “The Eve of St. Agnes” by John Keats. This does seem like an interesting text, but it is a literary work irrelevant to the historical documents I am seeking.
I believe I found a translation of Prudentius’ carmen here, on what appears to be another daily devotional/liturgical website. I am uncertain of how viable the text actually is.
Thankfully, St. Agnes (and her sister-saint St. Emerentiana) are found in Michael Lapidge’s The Roman Martyrs: Introduction, Translations, and Commentary beginning on page 348. Hopefully more will soon turn up as I continue searching and move forward with my research.