my goal is to be able to be compatible with hardware and software
targetted at the old system.
Would it be feasible?
Depends on the architecture of the target system and how compatible you want to be.
I don't know how to proceed with instructions 0x20 and 0x30 (RIM,
SIM). Is there a way in which I could "intercept" opcodes and
conditonally fed them into the Z80?
You could detect those opcodes by snooping the data bus while /M1 and /RD are active. When a RIM or SIM opcode is detected, switch the opcode to a different one using a mux between the data bus and Z80. This can't be done until the data bus stabilizes, but must be done before the end of the memory read operation (end of T2) or the Z80 will read and execute the original opcode (which is JR NZ for RIM and JR NC for SIM). If you need more time you could assert /WAIT to stretch the read cycle, but this must be done by the middle (trailing edge) of T2.
Your circuit could then feed more instruction bytes into the Z80 to make it do what you want (eg. JP to some code that emulates the RIM/SIM instruction) or send it a HALT instruction and then activate NMI to force a non-maskable interrupt.
This is bound to create some timing differences. Depending on the machine's architecture it might affect bus timing in a way that upsets other things such as the video display or DRAM refresh, or conflict with the existing interrupt system. The 8085 also has more hardware interrupt inputs that might need a different circuit and software for the Z80.
The emulated instructions will take much longer, which could be a problem for code that needs cycle accuracy (eg. tape read/write, bit-banged serial port) or execution within a maximum time frame (interrupt handlers etc.). The Z80 has different instruction timings and some flags are set differently, which could create more compatibility issues. Finally the 8085 has some undocumented instructions that some programs might be using, or they might have tricky code that relies on the CPU executing unimplemented instructions in a certain way (unlikely, but you never know...).
For these reasons I think that while handling RIM and SIM may be 'feasible', it probably won't be worth the effort unless the machine's hardware and software is unsophisticated and very tolerant of timing differences. 'Vanilla' CP/M code with only 8080 instructions would probably be fine, but for that you could just use the Z80 as is and patch the OS to handle the different instructions (perhaps by replacing RIM and SIM with RST instructions).
The sole reason is to reduce components whose production has ceased.
Doesn't seem like a very sensible reason. 'New Old Stock' and reclaimed 8085s are still available for those few boards that might need them. If you are thinking of producing new boards then you could use an FPGA with 8085 CPU core, which could be made 100% compatible as well as having enhancements. Programmable logic is the way forward for replacing out of production chips. On the other hand people who don't want their retro hardware 'sullied' by modern products probably want an original processor as well.