I hold a valid multiple entry Schengen visa issued by Spain, which has not been used so far to enter any Schengen country, Can I enter Romania based on such unused Schengen visa ?
According to the Romanian Ministry of Foreign Affairs, "the holders of Schengen visas with two or multiple entries, national visas or residence permits issued by Schengen Member States" can enter Romania without any additional visa.
They don't mention anything about there being any problem with you visiting Romania before any Schengen country on the visa. Since Romania is not a Schengen country, they let you in on the basis of the visa in the same way that they let in US citizens on the basis that they are US citizens, and thus don't need a visa. The only limitation that they mention is that your stay is limited to the duration of your Schengen visa. I recommend that you contact them or the Romanian embassy serving your home country to be sure about this.
Please note that:
For stays longer than 90 days visitors need to need to [sic] apply for a temporary residence permit (either before arriving into Romania or at least 30 days before the 90-day "no visa" stay expires).