If it were possible to have a broker hold the 25k in escrow and the trader uses the 5k in the account which is in his name, then the tax issues are straight forward.
i dont know many traders who would do this. 1) you cannot make a living trading 25k 2)they would need to surpass 12% a year to be profitable if someone is daytrading they need to be doing it as a job and 25k wont pay the bills.
What would prevent a trader from make one bad trade and wiping out the account? Suppose you have several traders long and a major terrorist attack occurs?
First try it with your cousins and siblings and friends and then you will find the problems practically and solve it.