Oh I understand, but it must be recognized. I assume that it was but wasn't sure.
I think you might not be correct though that SP can be considered under income tax. For example, I can't actually pay you in bananas in the US or UK, I must pay you in currency. You must only pay income tax on pay as income, so there is no income tax on non-currency payments. I think this is to protect people from unfair pay and to be able to account for it easily, and thus facilitate tax.
There is such a thing as "benefit in kind", which is probably what the bananas would count as if the relationship was employer / employee. I'm not to sure what a trade counts as, which in the case of witnessing would probably be running a service for an asset.
I'd also be interested to know whether SP qualifies as a currency even in the UK, since it is non-transferrable for example. You can only divest it into Steem at a fixed rate, suggesting that it's more like game or store points. Have you considered this?