I’m very late in this Velcorin thread and see that the subject has been extensively covered and answers provided but in case it is of any use as a summary I have set out my response under Kay’s questions below.
"Is this stuff a viable alternative to sulfites?".
It fulfils a hygiene function but not the anti-oxidation role of SO2. I have seen it discussed most often in its Brett control role mainly as an alternative to sterile filtering and like SO2 it is also used to prevent refermentation e.g. in the presence of residual sugar.
The latter quite often being connected to the former due to the increased potential for Brett to have its wicked way as high alcohol levels from very ripe grapes cause fermentations to stall as the Saccharomyces.cerevisiae starts to die.
Riper grapes not only provide the possibility of residual Brettanomyces.bruxellensis feedstock and the higher alcohol level but also a more ‘bug’ friendly lower acid [higher pH] environment which also lowers the efficacy of whatever SO2 is being utilised. However I was interested that Brettanomyces didn't appear to be listed in the official maufacturer's one pager [No.2 in Jeff's post above]
If SO2 usage is being minimised, Velcorin has been considered by some as a valid supplemental hygiene strategy, usually just prior to bottling particularly where fining and filtration are also being eschewed. Others may simply prefer its functionality.
A bit like adding the calcium channel blocker to the already utilised beta blocker and ACE inhibitor.
"Is it dangerous?"
Yes if the compound is in contact with the skin or, worse, inhaled or ingested. Specialised equipment is required for its use and AFAIK there are licensing requirements and strict protocols. So the danger is to the winemaker rather than the wine drinker.
"Harmless?"
Yes once used in wine if it is dosed appropriately since the product breaks down into compounds that are harmless at those concentrations. As dimethyl dicarbonate it is toxic - as above.
"Somewhere in between?" . As above
Velcorin was banned in the EU until fairly recently although perhaps unsurprisingly I haven't seen anyone claiming to be using it in the EU. It would certainly seem highly unlikely that anyone with proclaimed 'natural wine' objectives would use it although ‘low/no fining and filtration’ and low SO2 usage might be considered part of ‘natural’ winemaking so there may be some ambivalence.
Some more detailed information from Scott Labs in the USA including reference to equipment costs and training requirements
HERE