The standard TOSLINK Spec (without utilizing the extensions) was commercialized in the 1980s and only has a bandwidth of 3.1 Mbits/s at the ground floor. This can't quite manage 5.1 uncompressed audio with overhead, especially at >= 48k/16. So this isn't a DSP issue, its a bandwidth issue. The TOSLINK hardware link layer just can't manage the bandwidth I'm afraid. Extensions to TOSLINK are available to increase bandwidth, but they aren't widely supported, especially on audio equipment.
It can manage 5.1 with DTS/DD because of the compressed bitrate, but clearly this isn't an option when latency and quality are important.
Further more, TOSLINK has known clock recovery issues that give rise to jitter. Fairly acceptable for 1983, but 30 years later we have better options.
The MPB is perfectly capable of 5.1 uncompressed using the HDMI interface, which has superior bandwidth, FEC, and clock recovery when compared to TOSLINK.
In addition, there are plenty of external professional options available for this problem (pro tools, etc)
Bottom Line: You won't see AV professionals running a TOSLINK out of their MPB for any serious project.