Ibeka (now spelled 'Ibika') is indeed in the Eastern Cape, not far from Butterworth. Back in 1877, it was in the area known as 'Fingoland'. Ibeka was used as a military base during the war, and was the scene of a battle in September 1877.
The availability of Janssens' records, if any, would depend on which unit he was in. If he was a volunteer, then you'd need to find out which unit. As there was no centralised defence administration at that time, the records were kept by the units. If the unit still exists, it may have records. If it no longer exists, there's a small possibility that records can be found in the Archives in Cape Town.
If, however, he was in the Frontier Armed & Mounted Police, there may be something among the FAMP records in the Archives.