BaarakAllaahu feekum. May Allaah make the affairs of the brothers who are incarcerated easy for them and establish them firmly upon His Deen.
As for this question, then last night, the 19th of October, 2003, I asked our Shaykh, Aboo 'Abdir-Rahmaan Taariq as-Subay'ee, about this issue. The Shaykh -hafidhahullaah- said that no, this is not permissible, Maghrib and 'Ishaa' cannot be combined for the sake of praying in Jamaa'ah (congregation).
And Allaah knows best.
Aboo Sufyaan 'Uthmaan Beecher Kuwait أبو سفيان عثمان بيشر الأمريكي