The requirements for candidates in the London elections are given here:
http://www.londonelects.org.uk/info_for_candidates.aspx
(It took me quite a while to find that page!)
In brief, it isn't cheap or easy to stand.
To stand as a candidate for Mayor of London requires a £10,000 deposit and you also need:
330 signatures from London electorate - 10 from each borough
Which is an awful lot of signatures to gather!
To stand as a London-wide candidate for the London Assembly requires a £5,000 deposit. I can't see any info about how many signatures are required for that. Does that mean none are needed? This seems unlikely.
To stand as a constituency candidate for the London Assembly requires a £1,000 deposit. Again, I can't see any info about the number of signatures required. Presumably this is the lowest-visibility option, as the Liberal Party's name would only appear on the ballot paper in one constituency, rather than across London.
Nominations open on 18th March and close on (I think) 28th March.
It could be done, but it would take quite a while to raise the deposit and gather the necessary signatures, especially if we only have a smallish number of activists in the capital. Maybe it would make more sense to build up some areas of strength in individual London council wards, thus providing a basis for a stronger London Liberal organization, and then contest the elections in 2012 if things are going well by then?