UVM Backdoor access

[Bazinga24]

Hi all,

I have a scenario where I'm performing a backdoor write on some flops on the dut. I have a register with certain bits being used, linked to corresponding flops on the rtl. 

For example : say you have a 32 bit uvm_reg a_test [31:0] and a_test[1] is linked to flop top.dut.a, a_test[3] is linked to top.dut.b and you want to perform the following operations on them. 

assign a_test[4] = top.dut.b             ---step 1

backdoor write deposit top.dut.b = a_test[3]           --- step 2

How can that be done using uvm backdoor access support routines? (For a_test[3] I can add_hdl_path_slice to top.dut.b, but how can I perform the first step without directly using the hdl path? )
I am stressing on using uvm support routines because the code has to run on an emulator.