See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-778.md#text-encoding for the relevant part of ENR spec.

1. Adapt base64 encoding/decoding functions to support URL-safe encoding (basically use - instead of +, _ instead of / and don't pad with =)
   https://github.com/ethereum/aleth/blob/master/libdevcore/Base64.h

2. Add method to ENR class returning string with text form of ENR (serilize to RLP and base64-encode it)

3. Add enr field in the output of admin_net_nodeInfo

   aleth/libweb3jsonrpc/AdminNet.cpp

   Line 27 in 6890d9a

   Json::Value AdminNet::admin_net_nodeInfo(std::string const& _session)

   
   You'll need to extend dev::p2p::NodeInfo struct for that and fill it in the method Host::nodeInfo() with Host::enr().