This is a tricky one, and difficult to answer with any certainty.
The UKBA currently say that:-
Maintenance may be provided by either:-
The applicant with their own funds or with funds available to them
OR
The sponsor
OR
A combination of applicant and sponsor funds
OR
Third party support (from family members). The ECO may request evidence (for example, original bank statements over at least three months) of the third party's assets. Third party support is not precluded from consideration under the maintenance requirements relating to a spouse.
Immigration Directorate Instructions - Chapter 8, Annex F
5.1. Assistance from other sources
"A couple or other applicant who is/are unable to produce sufficient evidence to meet the maintenance requirement may provide an undertaking from members of their families that those members will support the couple/ applicant until they are able to support themselves from their own resources.
Third party support is not precluded from consideration under the maintenance requirements relating to spouses, civil partners, fiancé(e)s, proposed civil partners, unmarried partners, same-sex partners, children, parents, grandparents and other dependent relatives of sponsors who are settled in the UK.
The caseworker will need to verify and assess an offer of third party support in order to determine whether an applicant satisfies the requirement that he/ she can be adequately maintained in the UK without recourse to public funds. The caseworker may request evidence (e.g. original bank statements over at least three months) of the third party’s assets.
It is open to the caseworker to ask a third party offering long-term support to become a joint sponsor and to give an undertaking (under paragraph 35 HC 395) to underwrite his commitment."
The ECO must look at the question of whether third party support is sufficiently well-evidenced and reliable-looking to take into account when considering the question of maintenance
BUT..... the immigration rules above mention that a
Sponsorship Undertaking Form
may be required/requested.
The big problem here is that this form appears to indicate that UK residence is required.
I think it's better to get this clarified in writing from UKBA