According to Russian Space Web, separating the orbital module prior to reentry burn was not only possible, but it was the normal procedure up to 1988:
Originally, the habitation module [i.e. orbital module] was designed to separate from the descent module, SA, prior to the reentry maneuver at the end of a mission. However in the wake of a nearly disastrous incident in 1988, the habitation module would only be jettisoned after the braking maneuver, despite the extra propellant required to push the whole spacecraft off the orbit. Otherwise, any major failure of the braking system would strand the crew inside the cramped descent module with no access to toilet and other critical life-support systems.
Here's the description of the TM-5 incident:
After rather uneventful seven-day visit to Mir, a guest cosmonaut from Afghanistan Abdul Akhad Momand boarded the Soyuz TM-5 spacecraft for a trip home, accompanied by an experienced commander Vladimir Lyakhov, returning home after a long-duration mission to Mir. Immediately after undocking, a combination of human errors caused the Soyuz to spin around, while still in proximity of the station. Fortunately, the commander was able to stabilize the ship quickly and safely depart the station. Sometime later, as planned, the Soyuz jettisoned its habitation module in preparation for the deorbiting maneuver. (Separating the module would allow to save fuel for deorbiting maneuver).
However, 30 seconds before scheduled braking maneuver, the orientation system onboard the spacecraft failed causing a seven-minute delay in the engine firing. When the engine did fire automatically, Lyakhov immediately [shut] it down, since he had no idea where the new reentry trajectory would take the craft.
One orbit later, the crew made a second attempt to deorbit their Soyuz. However, this time, the automatic system confused by previous emergency instructions from the ground, not only [shut] down the engines, but also launched a countdown for the separation of the propulsion module with all its vital systems including braking engine itself. If Laykhov did not manage to suppress the countdown, the automatic system would separate the reentry capsule from the propulsion module and most certainly doom the crew.
After the ordeal, the Soyuz TM-5 and its crew circled the Earth for 24 hours, without toilet and water left in the jettisoned habitation module. Another deorbiting attempt was made on September 7, 1988, when everything worked perfectly and the crew landed safely.
The Soviet designers learned from the lesson: never again the habitation module, with all its vital systems, would be separated from the Soyuz until deorbiting maneuver had been successfully completed.
Because space mission designers generally like to keep their options open, I would guess that only policy changes were made in the wake of that incident, and that it's still technically possible to separate the OM prior to separating the service module, but I don't know for sure.
