For guidelines

* Transaction version numbers will be increased, if possible

* Transactions with unknown/large version numbers are unsafe to use with locktime

* Reasonable notice is given that the change is being contemplated

* Non-opt-in changes will only be to protect the integrity of the network

Locked transaction that can be validated without excessive load on the network should be safe to use, even if non-standard.

An OP_CAT script that requires TBs of RAM to validate crosses the threshold of reasonableness.