How to inspect global temp table from outside the transaction it is created in
File this under "just because you can, doesn't mean you should"

If you generate a bind token in the first session, and somehow publish it, you can join its transaction from another session. 

Eg from spid 61:

if @@TRANCOUNT &gt; 0 rollback
go
begin transaction

select *
into ##t
from sys.objects 


declare @bind_token varchar(255);  
exec sp_getbindtoken @bind_token output
declare @bind_token_bin as varbinary(128) = cast(@bind_token as varbinary(128))
set context_info @bind_token_bin

--rollback


then from another session

declare @bind_token varchar(255) = (
    select cast(context_info as varchar(255))
    from sys.dm_exec_sessions
    where session_id = 61
)

exec sp_bindsession @bind_token
go
select * from ##t