Case:
[ Subnet A , `192.168.2.0/24`, Padavan firmware based internet gw ]

[ Subnet B , `192.168.1.0/24`, Padavan firmware based internet gw ]

Host from subnet A (2.155) is connected via VPN (possible options: PPTP, OpenVPN, L2TP w/o ipsec) to subnet B, and receives address, saying `1.245/32`

In subnet B exists host (`1.10/32`) which sends multicast datagramms to `224.0.0.50:9898` ; On router I see them with

`tcpdump -i br0 -c 10 dst host 224.0.0.50 and port 9898 and multicast`

`13:46:54.345369 IP 192.168.1.10.4321 > 224.0.0.50.9898: UDP, length 135`

I am looking for solutions, to receive/forward those broadcast messages, so they could be seen by hosts, connected via VPN

On router B, which is Padavan firmware based, I have, and limited to udpxy, igmproxy utilities, if needed. 

On client host, I am debian based, and generally not limited in tools.

Datagrams are proprietary protocol, i.e. not a iptv or video stream.

Any ideas are welcomed.

[UPD] Additional info - per discussion in comments

That's a very specific hardware device, which is not very chatty in ethernet terms (saying max 1-2 datagramms in 5 seconds), thus for sure should be pretty forwardable. Unfortunately, It sends status updates purely via broadcasting. in Subnet A do exist similar device + control software. Thus I am looking for a way datagramms broadcasted to 224.0.0.50:9898 in subnet B to re-appear in subnet A. May be with help of some tool. May be smcroute, may be udpxy, maybe igmproxy