In fact, this task is perfectly well fulfilled by merchants and service providers just accepting a certain currency (Bitcoin in this case) as a means of payment for their goods and services. It could be said that any currency which is not directly backed up by a specific asset is backed by all the goods that can be bought for it.
Yes, but in the present situation there is a problem: most merchants use a solution like BitPay where the price of the good follows the BTC/USD, BTC/EUR etc. exchange rate. BitPay, as far as I know, guarantees a fixed price only for 15 minutes. If merchants could extend this period to 24 hours, they could really "back" the price in the case of a crash.
On the other hand there are merchants (e.g. in OpenBazaar) that accept fixed prices in BTC for their goods but can end the product sale every time. That is also only a limited "backing" of the price because in the case of a crash most merchants would retire their products until the dust has settled.
So the "price guarantee" for 24 hours should be an option widely available for merchants, in most payment platforms.
how are you going to implement backing up of a decentralized currency in practice?
It would be - a decentralized backing ecosystem
My proposal is to provide the "backing option" for plugins for most Bitcoin merchant solutions (e-commerce platforms, wordpress commerce plugins etc.) and integrate it into centralized payment providers like BitPay and also into decentralized solutions like OpenBazaar (although that is already a little bit more complicated because we'd need an "oracle" for it like in Bitshares).