B. liniflora cuttings worked for me. This time round, I had the chance to try out another species when the whole pot of B. guehoi fell over and a branch broke.
About 2 weeks later...
Close-up of roots
New plant potted up. And ready to go to another parent.
Since it was a success, I decided to save some the declining B. liniflora as well as take more B. guehoi cuttings. Will be trying it on B. 'Goliath' and B. rorida once they are almost done with flowering.