There are not perfect market, none traditional neither green market could be considered as perfect market, information gap is always difficult to solve under so call traditional market. Many regulation try to deal with these on traditional markets, but also special regulation,,,call self-regulation mechanism try to reduce the uncertaintly under green market results. I think on both cases you will arrived to the same conclusions, markets are imperfect institutions (traditional and green ones).

Regards

Other than the model applied in corporate finance WACC, when we prepare feasibility studies for listed and unlisted companies we use free interest rate and add both economic and political risk based on risk assessment of the country as well as companies business risk. This judgmental approach reflects appropriately the discount rate for such studies. However, most of the comments which I see as a feedback to such approach lies in discussions of the calculations of the political and economic risk of the country. As to the companies business risk one of the elements which are being used is borrowing interest rate of the companies and the difference between interest free rate and borrowing rate. As interest free Islamic finance system I would consider cost of borrowing applied for Islamic types of Morabahat and modarabat

What if for the risk free rate you determine a base index for islamic securities (which should be provided by several IFIs) with the least volatility that still generates positive returns as cost of equity and then you proceed with the assumption of 100% Equ Capital Structure? You might need to adjust for general market movements to lower the risk of a negative net present values. 

If the expectations hold true, the higher yield of equities could compensate for the lack of debt financing. 

However, if you calculate historical discount rates that have been realized, I would expect that you would find, that companies that only use financing methods that are conform with Islamic norms, would have already paid a higher risk premium.

Otherwise you would need to find comparable industry benchmarks for the respective company.