It can be done. My Bro, a student, borrwed 20k interest free for 4 years medical loan as a deposit for an apartment, of which I am guarntor on his mortgage as he has no income.
Tenant pays bulk of mortgage, folks help pay the mortgage for his share instead of giving him rent as they would be doing otherwise.
Only draw back is if there is a crash you may be paying back a mortgage of higher value than the property is worth.
I do not see the numbers stacking up on a buy to let though with a normal personal loan as the repayments on this over say 5 years coupled with the mortage, less tax on rental income will make this prohibitive.
Crunch the numbers seriously....I cant see it working for a buy to let.