Not even using VGMTrans to get MIDI sequences would actually work, if you were to be specific.
Take Hot-Head Hop from Donkey Kong Country 2 for example.
The MIDI sequence shows a length of 1:52. However, the length of the MIDI does not take into account MIDI effects that are added during the song to channels, which are not reset. In this example, listen to the cymbals at the beginning. This same channel is also used for other instruments during different parts of the song. At one point, an echo effect is added. This however, is never reset, so after the loop, the cymbals sound like they are being played twice.
Another example also from DKC2. Mining Melancholy. One of the many channels that play a rhythm of metallic clinging (the one that has the simple sound of CLING cling CLANG clang, in quavers) is also used for the low-pitched string-pad type sound at the end. This instrument is not reset at the loop point, so, if you listen carefully, instead of hearing that particular cling clang sound, you instead hear a series of short low-pitched humming sounds... when mixed with the rest of the tune, it sounds more like "wom". https://www.youtube.com/watch?v=BkRvgb27xnU
The DKC series is full of this.
In short, the sequences only really tell the length, purely based on notes... and do not take events into account, if the start point of the sequence is AFTER certain events, which leads to instruments not being reset to what they should be, echoes being added and not removed, etc.
If you find a .spc with an extended ID666 tag, it can have loop info, this shows up in the Extended tab in SNESamp and I think it should be in the "other" properties in fb2k.
You don't need to use BrawlBox to find loop points. If you have vgmstream for foobar, you can just right click, go to properties, open the properties tag, and scroll to the bottom for loop points.
Are the loop points in the extended ID666 tags even accurate? For example does Hot-Head Hop have a loop point between around 1:45 (start) and 3:31 (end) or near enough?
There's nothing about the file format that forces the loop tags to be accurate, any more than it forces the title to be accurate. So, no, in general. For particular sets, who knows?
@Kurasukun: I think the point about BrawlBox might have been that you can fiddle with the loop points interactively when trying to loop a file without explicit loop points (like a recording of an SPC), which you can then use in other ways (like adding a loop to an ADX) once you find it.